Abstract: The bias in estimating out crossing rate (T),one of the most important mating system parameters, was discussed in this paper.The conclusion is as follows: the exact value of out-crossing rate (or the theoretical value of outerossing rate)[Te] is iower than the one estimaled by seed analysis [To],i. e. To>Te, As a result,the estimated difference between two individuals or two populations is smaller than the exact one,i. e.if Tel>Te2,then To1—To2