Abstract: A shear-lag model is presented for analysis of load concentration of fibers adjacent to a crack in unidirectionally fibei-reinforced composites.The analytic solutions of stress concentration factors of fibers adjacent to a crack-tip are obtained by using this model.The maximum stress concentration factors are calculated of the fiber at the tip of a crack,and the numerical results show little difference with Hedgepeth's results.In addition,the concept of the average stress concentration factor is presented,and the affected length of the crack-tip fiber is defined by means of the value of the stress concentration factor.Its physical meaning is clear,and it increases gradually with expansion of the crack,which is in accordance with the actual case.Based upon the previous analysis,statistical analysis of failure of unidirectionally fiber-reinforced composites is done by;applying the method of the ultimate crack core model^2,3.The results are close to the experimental data,and those obtained by using the average stress concentration factors approach the experim ental data more closely.The ccmparison of the results here with the experimental data chows correctness of the method of the ultimate crack core model and feasibility of the affected ogrl and the average stress concentration factor in the analysis of crack propagetion.