Three dimensional (3D) photonic crystals (PhCs) are important for many applications such as photon confinement, optical switching, bio-sensing etc. Their unique property arises from the periodic arrangement of micro/submicro-meter particles. There is a lack of simple method to fabricate the same. Here we present a simple method to fabricate uniform, large area 3D PhCs using polystyrene (PS) nanospheres by self- assembly method. PS nanospheres (150-200 nm) were synthesized using styrene monomer by wet chemical method. Size tunability of PS nanospheres has been achieved by changing the cross linking agent concentration. Finally self-assembly was utilized for PhC fabrication. © 2012 OSA.