Trichoderma harzianum is an important commercial biocontrol fungal agent. The temperature has been shown to be an important parameter and strain-specific to the mycelia growth of fungi, but less report makes the known of the mechanisms in T. harzianum. In our study, a 6-h treatment of heat increased the thiobarbituric acid reactive substances (TBARS) and nitric oxide (NO) concentration in mycelia to 212 and 230 % the level of the control, respectively. The exogenous NO donor sodium nitroprusside (150 mu M) reduced the TBARS concentration to 53 % of that under heat stress (HS). At the same time, the NO-specific scavenger at 250 mu M, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-1-oxyl-3-oxide, prevented the exogenous NO-relieved TBARS accumulation under HS. The increased NO concentration under HS was reduced 41 % by the NO synthase (NOS) inhibitor L-N-G-nitroarginine methyl ester, but not the nitrate reductase (NR) inhibitor tungstate. Our study exhibited that NO can protect the mycelia of T. harzianum from HS and reduce the oxidative damage by enhancing the activity of NOS and NR.