we're working on ubuntu platform,
so still want to build a same type of rootfs for embedded board.

to save space, we need rootfs small.

but need that apt-get in case we need to install stuff.

any hints ?

How small? Honestly, I would use a distribution better suited to embedded devices than Ubuntu; that's really not what it's designed for.

is it possible to get it under 100M ?

the rootfs we built is painful, it used busybox and lots of things are not working nicely.
we have 2 eth ports, but not NetworkManager, etc.

100MB compressed or in uncompressed state? Also, things like NetworkManager are designed for full blown systems, not really embedded machines. It is possible to get quite small systems with Ubuntu, if you use debootstrap for installing a really minimal system (using the --variant=minbase option) and then carefully add the packages you need, with the installation of recommends and suggests disabled. But only the minimal base system (which only contains the essential packages and apt) will be about 200MB uncompressed and you also have to add the kernel to that.

our case is uncompressed,
200MB is larger than what we can have, but i can understand it's already tiny for desktop.

Ubuntu really is not the distro for such a restricted embedded system.

What board is this?

What is the goal of this system in the first place? What does it need to do? Determining how small the filesystem image needs to be will greatly depend on the required functionality. For example, if you need X, things are going to take a lot more space than if you don't.