The magnetic hysteresis behavior of crystalline Co50Pt50 films has been studied in samples with thickness in the range of 100-2000 Å. The room temperature coercivities of the as-grown films are of the order of 100 Oe. After annealing at 650°C the coercivities vary between 400 and 4300 Oe with the higher values obtained in thicker specimens. The high coercivities obtained are attributed to the hard magnetic phase CoPt with an ordered fct structure that is formed after annealing. The variaton of coercivity with thickness is examined and related to the crystal structure evolution and to grain size effects.