Abstract: Polyploid breeding is proved to be advantageous over other methods of breeding in Populus. Given the problem that cross breeding was very difficult in P.tomentosa when using those serious abortive gametes as female parents, P.tomentosa clones 3119, 3532 and 8212 with higher seed setting rate from many years of field observations were selected as female parents, and P.alba×P.glandulosa YX1 as male parent. Before chromosome doubling was carried out, female flower branches should be cultured in water while their pistil development was approaching receptive period, which can overcome the problem that gamete abortion was likely to happen because of malnutrition. During chromosome doubling of embryo sac, female catkins, 4, 16, 28 and 52 h after pollination, were treated for continuous 2 and 4 h by 39 and 42 ℃ high temperature exposure, respectively. And control groups were set without high temperature treatment. In addition, female catkins of P.tomentosa clone 3119 with the same conditions were fixed for paraffin sectioning, sections were observed and the percentages of embryo sac at different developmental stages were statistically counted for determining the correlation between chromosome doubling in embryo sac of P.tomentosa clone 3119 and the development of embryo sac. The results showed that, in total, 458 triploids were produced from the treatments of three cross breeding. Among them, 211 triploids were derived from the cross breeding between clone 3119 and P.alba×P.glandulosa, and the average rate of triploid production was 57.97% with a total of 230 triploids came from the cross breeding between clone 3532 and P. alba × P. glandulosa, and the average rate of triploid production reached 65.71%. However, only 17 triploids were screened from the cross breeding between clone 8212 and P.alba×P.glandulosa, with an average rate of triploid production of 70.83%. No hybrid triploids were obtained in control groups. From the view of effective treating period, P.tomentosa clone 3119 female catkins after pollination 4-64 h were in the three mitosis periods of embryo sac development. There was a certain degree of asynchrony in different catkins and different ovules of the same catkin, suggesting that the obtained hybrid triploids by high temperature treatment were from chromosome doublings of embryo sac. Considering the temperature and duration of treatment, the temperature of 39, 42 ℃ and the duration of 2, 4 h are all suitable to P.tomentosa embryo sac chromosome doubling, and little difference was observed between 39 and 42 ℃, the same for duration between 2 and 4 h. Observations of growth traits in hybrid progenies revealed that the average height and ground diameter of triploid seedlings were larger than those of diploid individuals. Our findings interprete that, chromosome doubling of embryo sac with high temperature exposure is an efficient method for inducing triploid poplar hybrids, which has important values in genetic improvement of P.tomentosa.