A fungi HY12, possessing high cellulose activity, was identified by 18S rDNA sequence and morphological observation. The sequence data was compared with the published sequences of relative fungi strains in Genbank. The results showed that the homology of the 18S rDNA sequences of strain HY12 with those of several Thichoderma strains was more than 99.0%. As shown in the phylogenetic tree, strain HY12 and Trichoderma viride constituted a branch and they constituted a high-order branch together with Trichoderma harzianum. Besides, the strain HY12 was similar with Trichoderma viride in morphological character. So the strain HY12 belonged to the species of Trichoderma viride.