In order to elucidate the origin of magnetic properties and microstructure of electroless-deposited CoNiP films with perpendicular magnetic anisotropy, their initial growth processes were investigated mainly focusing upon the effects of bath factors. It was clarified that coercivity and grain size of the films changed widely with an increase in the ammonia concentration in the bath without varying the average film composition and crystal orientation. Detailed analysis of the growth processes clarified that an increase in ammonia concentration in the bath inhibited the growth of each crystallite, resulting in small grain size and coercivity, whereas it hardly affected the deposition activity of the catalytically-treated substrate surface and average film composition.