Spinel ferrite CoCrxFe2-xO4 nanoparticles over a compositional range 0 < x < I were synthesized using a reverse micelle microemulsion method. This method provided excellent control over the composition and gave a reasonable size distribution. Upon increased Cr substitution, the blocking temperature, saturation magnetization, remnant magnetization, and coercivity were all found to decrease. The compositional influence upon the magnetic properties is consistent with the effects on the magnetocrystalline anisotropy energy by weakened coupling between electron spin and the angular momentum of electron orbital (L-S coupling). The results from size dependent magnetic studies on the nanoparticles with a composition of CoCr0.5Fe1.5O4 were found to agree well with the Stoner-Wohlfarth model.