This study is concerned with deep trap densities and energy levels in CVD Si3N4 layers deposited on Si substrates at 700°C with various gas flow ratio NH3/SiH4. As the ratio NH3/SiH4 increases from 10 to 1000, the trap density decreases from 8 × 10l9 t0 2 x l019cm−3, and energetical distribution of trap states becomes lower and/or electron emission probability from trap states less. The results are discussed in terms of chemical and optical properties of Si3N4 film revealed by infrared absorption, Auger electron spectroscopy, and ellipsometry. It is shown that (i) Si dangling bonds create deep trap states and (ii) 0 and H atoms act as terminators to decrease trap density.