It is well known that the spin of proton is 1/2, but its origin is poorly known. Past measurements on quark helicity distribution show that quark spin carries only 20-30% of the proton spin. This is a long-standing problem called "proton spin puzzle". In order to solve the problem, we are measuring gluon spin contribution at Relativistic Heavy Ion Collider using polarized proton beams (RHIC-spin). I will show recent results of RHIC-spin and discuss how the results improve our understanding of proton spin. I will also mention future upgrades of RHIC-spin.