Abstract: Mitochondrial cytochrome b (cyt-b) genes were sequenced from one individual of N.coucang and two individuals of N.pygmaeus.By comparing with the cyt-b sequences of three species in Lorisidae,the divergence and base substitution among these sequences were analyzed.The phylogenetic tree,in agreement with the fossil and morphologic data,was reconstructed with Daubentonia madagascarie nsis as the outgroupThe base composition of N.coucang and N.pygmaeus showed the same character as that of other mammals.The Tv/Ts ratio between the two Nycticebus species was as 6 times as other pairwise comparisons.The lo w values (<0.1) of Ka/Ks ratio implied a strong selective constraint against th e nonsynonymous sites of the cyt-b genes from the species in Lorisidae.Based on the fossil data,the transitions and synonymous mutations at the third codon position that have shown a linear relation with time,the estimations of 3 Myr (m illion years) and 6 Myr were the divergent time between N.coucang and N.py gmaeus,and between Lorisines and Galagines groups,respectively.