Two strains of Bacillus thuringiensis var. thuringiensis and Bacillus thuringiensis var. kurstaki have been tested for insecticidal toxicity against Sedge stem borer (Bactra venosana Zallev) at Nga Son district, Thanh Hoa province. In lab. condition, on 3th instar larvae, strain of B. thuringiensis var. kurstaki after 5 days of treatment gained mortality of 20.59 percent,67.65 percent and 82.35 percent at concertration of 2 x 10 exponent 6, 2 x 10 exponent 7 and 2 x 10 exponent 8 spore/ml, respectively. With strain of B. thuringiensis var. thuringiensis at 2 x 10 exponent 7 spore/ml, after spraying 5 days, mortality on 3th-4th instar larvae gained only 50 percent. After 7 days in field condition, insecticidal activity of B. thuringiensis var. kurstaki strain has been reduced about 50 percent its toxicity. This is the first research on exploration of insecticidal activities of Bacillus thuringiensis on Sedge stem borer Bactra venosana. Base on collected data, the authors find out the potential application of Bt-based biopesticides for control Bactra venosana.