Can I use my old ram in my new pc build?

This is the motherboard I'm getting Asus H81M-E Motherboard (Socket 1150, Intel H81, DDR3, S-ATA 600, Micro ATX, PCI Express 2.0, VGA, USB 3.0) and i was looking at corsair 4gb ram but I thought why don't I use my ram from my old lenovo h505s which is also 4gb will it work

Yep you can because your Lenovo h505s has DDR3 ram too which your motherboard supports.
Check which memory your lenovo has by visiting this site: http://www.crucial.com/...ucial.com/, they have a scanner which detects the memory type you have, it will come up with DDR3.

It uses DDR3-1600 memory so you need to make sure your motherboard is compatible with that

Yes you can only if the ram is compatible with the motherboard!