The fungus Trichoderma reesei QM9414 secretes a large amount of cellulase. Furthermore, QM9414 was treated with mutagens such as ultraviolet radiation and chemical compounds, and cellulase hyper-producing mutants were obtained. We investigated cellulase production mechanism of 3 mutants by analyzing the amounts of secreted protein, cellulase activity, and transcription of cellulase genes. The analyzed mutants were T. reesei X31, T. reesei PC-1-4, and T. reesei PC-3-7. We used Avicel and lactose as carbon sources. When we cultured these strains for 7 days using Avicel as carbon source, the amounts of secreted protein and Filter Paper enzyme activity of these mutants were 2.3 to 2.7-fold and 3.0 to 4.3-fold, respectively, higher than those of QM9414. In addition, RT-PCR analysis revealed that the transcription of cellobiohydrolase I gene and endoglucanase I gene of these mutants were increased compared with those of QM9414. Similar results were obtained using lactose as carbon source. These results suggested that mutations of these mutants were affected cellulase production at transcriptional level.