We studied some electrical properties of silicon crystals containing carbon, nitrogen, and oxygen. Nitrogen‐oxygen complexes are formed in nitrogen‐ and oxygen‐rich silicon crystals. However, we found that carbon suppresses the formation of nitrogen‐oxygen complexes. Moreover, new shallow effective‐mass‐like defects with g≂1.999, which includes carbon and nitrogen, were found. We could not observe the hyperfine interaction of nitrogen by electron spin resonance measurements even though the new defects contain nitrogen having nuclear spin. New effective‐mass‐like defects may be series‐like defects, because two effective‐mass‐like defects are observed. These properties resemble those of both nitrogen‐oxygen complexes and thermal donors.