We have studied magnetoelectric effect with cobalt ferrite-Pb(Zr, Ti) $O_3$ composites made by solid state reaction. The maximum magnetoelectric voltage coefficient, $(dE/ dH)_{max}$, increased with longer sintering time and higher volume fraction of the cobalt ferrite. The magnetic field for $(dE/ dH)_{max}$ became lower with increasing the sintering time and decreasing the volume fraction of the cobalt ferrite. The phenomena were explained in terms of grain size change, mechanical coupling efficiency, easiness of magnetization and polarization. We obtained the highest magnetoelectric voltage coefficient of 0.174V/cm-Oe, which is about 30% higher than the best value reported.