The dependence of the seismic source spectrum on earthquake magnitude was determined with improved accuracy in the frequency range from 1 to 25 Hz using coda waves from local earthquakes. Separation of the source effects from the effects of scattering and attenuation was achieved by the method of Aki and Chouet (1975) assuming that the earth is a randomly heterogeneous medium with a uniform statistical property. Scaling laws constructed for seismic regions in Japan, California, and Hawaii show marked variations which may be attributed to the differences in the scale length of inhomogeneity and strength of the earth's crust among these regions. In the magnitude range from 1 to about 5, the stress drop increases in most areas from roughly 1 bar to about 1 kbar. The exception is Hawaii where the stress drop is very low and almost constant. The dependence of the stress drop on earthquake magnitude reflects the heterogeneous material properties in the seismic zone and is explained by a fault plane with barriers which may be skipped, unbroken, when the tectonic stress is low.