Abstract: An investigation of multiple crackiang in brittle reaction zone at interfaces of unidirectional composites with longitudinal tensile has been made by using three concentric circular cylinder model,and sheat-lay onolysis proposed by Cox.Both the cases of strongly and weakly bonded fiber/brittle reaction zone interfaces were considered.The crack sFacing was found to decrease with the increase of the brittle reaction zone thickness and increase with the decrease of the interface shear strength.