The electric field dependence and anisotropy of the impact ionization coefficients of 4H-SiC are investigated by means of the avalanche breakdown behavior of p(+)n diodes. The breakdown voltages as a function of doping density and the multiplication factors of a leakage current are obtained using p(+)n diode fabricated on (0001) and (11 (2) over bar0) 4H-SiC epitaxial wafers. The obtained impact ionization coefficients show large anisotropy; the breakdown voltage of a p(+)n diode on (11 (2) over bar0) wafer is 60% of that on (0001) wafer. We have shown that the anisotropy of the impact ionization coefficients is attributable to the anisotropy of saturation velocity originated from the electronic structure of 4H-SiC.