I don’t get it, what was the root password on my new Slackware Linux box. Since the partition use ext4 file system, all of my old linux rescue/emergency live CD have been failed to mount the partition. I turn back and read some standard manual on Slackware 13.0 installation DVD that must be hold the clue for resetting my root password.
After search a while, I found a part of manual on Slackware-HOWTO file, that pointing about how to reset the root password using Slackware first install CD or DVD. Here is The quote:
Passwords and security
----------------------
When choosing passwords for a Linux system that is connected to a network
you should pick a strong password. However, passwords only help protect a
system from remote trespassing. It's easy to gain access to a system if
someone has physical access to the console.
If you forget the root password, you can use the install disc to mount
your root partition and edit the files containing the password
information. If you have a bootable optical drive, you can use the first
installation CD-ROM or the DVD as a rescue disk.
At the prompt, you can manually mount the root Linux partition from your
hard drive ("fdisk -l" will give you a list) and remove the root password.
For example, if your root linux partition is /dev/hda2, here are the
commands to use after logging into the install disc as "root":
mount /dev/hda2 /mnt
cd /mnt/etc
Next, you'll need to edit the "shadow" file to remove root's password.
Editors which might be available include "vi", "emacs", "pico", and "nano".
"vi" and "emacs" might be more of an adventure than you need unless you've
used them before. The "pico" and "nano" editors are easy for beginners to
use.
pico shadow
At the top of the file, you'll see a line starting with root. Right after
root, you'll notice the encrypted password information between two colons.
Here's how root's line in /etc/shadow might look:
root:EnCl6vi6y2KjU:10266:0:::::
To remove root's password, you use the editor to erase the scrambled text
between the two colons, leaving a line that looks like this:
root::10266:0:::::
Save the file and reboot the machine, and you'll be able to log in as root
without a password. The first thing you should do is set a new password
for root, especially if your machine is connected to a network.
Here are some pointers on avoiding weak passwords:
1. Never use your name (or anyone's name), birthdate, license plate,
or anything relating to yourself as a password. Someone trying
to break into your machine might be able to look these things up.
2. Don't use a password that is any variation of your login name.
3. Do not use words from the dictionary (especially not "password" 
or syllables of two different words concatenated together as your
password. There are automated programs floating around on the net
that can try them all in a short time.
4. Do not use a number (like 123456) or a password shorter than six
characters.
The strongest passwords are a mix of letters, numbers, and symbols.
Here are some examples of strong passwords (but don't use these ;-):
*^5g!:1? ()lsp@@9 i8#6#1*x ++c$!jke *2zt/mn1
In practice, any password containing one or two words, a number (or two),
and a symbol (or two) should be quite secure.
And when I tried with my Slackware 13.0 installation DVD, it works! 
Except the text editor I used to editing /etc/shadow file. running pico give me some libc-bla-bla-bla error. But back to the standard text editor on any linux distros, vi is more than enough for do the task. Finish with editing the file and make sure to save it, after the reboot, my root user already password-less. It’s time to passwd-ing the root and everyone is happy 
Hope it help someone out there who face the same problems like me. For those who use Slackware, many unpredictable things that usually happened on our daily “linuxing” life, some of that just already mentioned on the Slackware Linux manual file.
So, when you don’t get it *especially on slackware*, RTFM!