We present a photonic crystal narrow pass-band optical filter, which is made of two pieces of photonic crystals with different photonic band gaps. Our numerical investigations show that this kind of structure is feasible for narrow pass-band optical filtering. The pass-band may be less than 0.1nm in the infrared communication frequency band. In ideal cases, the optical transmission in the pass band is very close to 1. This means a very low insert energy loss.