Abstract: In this investigation the probabilistic natures for fracture toughness of welded joints of A131 steel welded by LB52NS electrode were analysied by means of statistic treatment based on the test results of δ_cr,and δ_is,in the COD test of ductile/brittle transition temperatures.In addition the probabilistic nature of δ_cr was simulated with Monte-Carlo method also.It has been found that the probability density functions of either δ_cr,or δ_is can be expressed by weibull distribution with three parameters.Finally,the effects of test temperature and size of specimen on the distribution of the fracture toughness were discussed based upon the E(δ_cr) and D(δ_cr)concepts and available date were given.