Fabrications of ZnS nanocrystalline thin films at different substrate temperatures (T-S) of 200, 300 and 400 degrees C by means of pulsed laser deposition are presented. Thin film deposited at TS of 200 degrees C is in cubic zinc-blende (ZB) structure while those deposited at TS of 300 and 400 degrees C are in hexagonal wurtzite (W) phase. The grain size, surface roughness and bandgap of the films increases with increasing TS. The zinc vacancies and interstitials in the films increases while sulfur vacancies decreases with increasing TS. The variation of zinc and sulfur vacancies in ZnS films with TS is responsible for structural phase transition from ZB to W which causes the change in energy bandgap.