Cytochrome P450nor (P450nor) is a unique cytochrome P450 (P450), which plays a key role in fungal denitrification. P450nor is a nitric oxide (NO) reductase (Nor) found in eukaryotic microorganisms, which reduces NO to nitrous oxide (N 2 O) by directly accepting electrons from NADH or NADPH. Unlike other P450s, P450nor has no monooxygenase activity although it belongs to the P450 super family (CYP55A). Several P450nor genes have been cloned from the subdivision ascomycotina of fungi and the basidiomycetous yeast, indicating a wide distribution of P450nor across the subdivisions of fungi. The crystal structure of P450nor showed that this protein has a wider open cavity than other P450s, suggesting the possibility of direct access of NAD(P)H from the distal pocket, but not from the proximal site. Considering the different catalytic cycle of P450nor from that of P450 monooxygenases, important amino acids and clusters, which effect proton and electron delivery to the heme were investigated to explore its reaction mechanism and function. In this paper, we review the current advances in P450nor studies. The binding specificities of substrates NADH and NADPH to P450nor are also discussed.