Abstract: Factors affecting utilization of the thermo-sensitive male sterile line S1 in flax (Linum usitatissimum L.) were studied. Longya 10, a commercially used cultivar, was used as the pollen donor. The field experiments were carried out in 2004-2005 at three locations with distinct ecological environments. The petals of the male-sterile flowers opened normally and did not impose a barrier for pollination. The proportion of the fertile pollen increased with the increase of the temperature during the florescence. At Lanzhou where the average temperature during the florescence was 22 ^oC, the fertile pollen of the male-sterile plants was 10.94% on average and above 10% for 17 days. At Yuanmou where the average temperature during the florescence was 14 ^oC, the fertile pollen was 5.29% on average and below 2% for the first 12 days of the florescence. It may be possible to make use of this thermo-sensitive feature of this male sterility in hybrid production by maintaining the male-sterile line where the temperature during the florescence is relatively high and producing the hybrid seeds where the temperature during the florescence is relatively low. In contrast to the earlier belief that flax pollen could not travel in the air, flax pollen traveled as far as 2 – 3 m in the air in this study. An estimation of 92% hybrid seeds were harvested from the naturally pollinated male-sterile plants when 10 rows of the male-sterile plants were inter-planted with two rows of the pollen donor plants.