In the present paper investigation of electrical characteristics of p(--) - 3C-SiC /n(+) - 6H-SiC heterojunction grown by sublimation in vacuum was done. Capacitance-voltage characteristics of this heterodiodes show that the grown hetero-pn junction is abrupt. The electroluminescence spectrum (EL) of these diodes shows green band close in spectrum position to band of free-exciton annihilation in 3C-SiC. But the maximum position of this line was shifted to the short- wave region of the spectrum with about 0.05-0.07 eV in contrast with the typical position of exiton band in 3C-SiC. This band is also close to spectrum position of so-called "defect" EL in 6H-SiC, but has another temperature dependence of the intensity. Finally it was concluded that green band in p(--) - 3C-SiC /n(+) - 6H-SiC diodes is connected with free-exciton annihilation in 3C-SiC and shift in maximum EL position may be connected with quantum - size effects at 3C-SiC/6H-SiC hetero-boundary.