We report the results of a CCD investigation of stars in a selected region near the center of the intermediate age galactic cluster NGC 7789. A CM diagram is presented for a sample of 1122 stars, which should be complete down to V=20. On the ground of recent theoretical evolutionary models, simulations of both the colour-magnitude diagram and the cluster luminosity function are discussed under different assumptions about age, mass function and binary population. We find that the faint main sequence (V<15) appears rather depopulated of stars, with a distribution of luminosity lower than that expected in the case of a Salpeter's mass function. The distribution of post main sequence stars in the various evolutionary phases is well reproduced by classical models (i.e. with negligible overshooting of the unstable convective layers). Data are best fitted by models having an age of 1.2Gyr. Derived values for distance modulus and reddening are in agreement with previous evaluations. We obtain V-M_v_=12.3+/-0.2 and E(B-V)=0.32+/-0.03.