Effect of post annealing on superconductivity of MgB2 thin films has been investigated. On increasing the annealing time (at a temperature of 900℃) from 30 mins to 120 mins, the grain size was found to increase but the transition temperature and critical current were seen to be reduced. This is attributed to increased mis-orientation of grains caused by interaction between the film and the substrate. Evidence of film-substrate interaction was obtained from angular dependence of resistivity (ρ), measured on films annealed for 30 mins. These measurements showed increased pinning for vortices parallel to the ab-plane that is ascribed by film-substrate interaction.