Abstract: Abstract: For analysis on the heterosis and combining ability among Bt transgenic cotton high epression lines. The crosses were made by NCII design, using four elite commercial cotton culrivars as female parent and nine transgenic Bt cotton protein high epression lines as male parent, to analyze the competitive heterosis and combining ability effect for 36 combinations.The results showed that the heterosis for yield and yield traits were obvious in most combinations. of which lint and seed cotton yield were with more competitive advantage, both of them were 16.1% and 6.8%, respectively lint and seed cotton yield competitive advantage rate reached 83.3% and 80.6% respectively. There were positive heterosis in lint percentage,boll weight and boll number in most F1 hybrids; leading to average hetersises in three traits for 3.7%,2.0% and 0.8%, respectively. The results of variation analysis of combining ability showed that there are significant difference for general combining ability among nine transgentic Bt cotton protein high epression lines. three F1 hybrids taking on elite special combining ability were selected,