And after windows is installed, dual boot no longer work because windows installer overwrited the mbr. I didn't want to install ubuntu again, and luckily, I found this: Dual Boot Ubuntu and Windows.
In short, we can fix the grub with following steps:
- Boot with ubuntu live cd
- Mount the ubuntu installation file system
- Reinstall grub with "grub-setup -d ubuntu_installation_path/boot/grub" command