[en] Irradiation of Trichoderma harzianum with 0.5-8.0 k Gay of gamma ray revealed 41 strains resistant to 10 ppm propiconazole or benomyl fungicides and two strains (23/03-7 and 27/08-1) were resistant to 10 ppm of both fungicides. After these two mutant strains were repeatedly irradiated with gamma ray, 74 mutant strains were obtained. Among these, three mutant strains, used as seed treatment effectively protected tomato seedlings from the infection of Pythium apanidermatum with significantly higher surviving seedlings than the Pythium inoculated control. The higher root colonization of mutant strains was obtains from strains 03/7-113, 03-/7-114 and 08/1-11. Rice seeds (RD 17), previously soaked in water for 24 ht were placed in spore suspension of T. harzianum prepared from 1 kg of fresh culture of 81 mutant isolated derived from single or double irradiation with gamma ray in 50 1 of water for 30 min. Two mutants including 23/03-7 (derived from single irradiation) and 03/7-134 (derived from double irradiation) provided percentages of root colonization by 29.63 and 25.93, while growth-promoted roots were 12.48 and 12.65 cm in length. respectively. These two strains were tested in rice field by treating pre-soaked seeds with Trichoderma suspensions for 30 min and incubated for 24 hr before sowing. Detection of root colonization by T. harzianum at 35, 45 and 102 days after sowing revealed that all Trichoderma strains effectively colonized rice roots at all stages of growth, particularly two mutants completely colonized rice root at 102 days after sowing. After harvesting, a mutant strain 30/7-134 increased rice yield to the maximum level at 29.65% over a control, while the percentage of fertile-seeds and it's seed weight, total seed weight, fertile-seed weight were significantly higher than a control. However, all Trichoderma strains provided the potential increases of rice yield over a control. Strain 03/7-134 significantly reduced percentage of dirty-panicle diseased seeds lower than all treatments.