Spew fillet is common in co-cured composite single lap joint. It plays an important role in the safety of the joint. But few attentions were paid to the subject in former investigations. In this paper, co-cured composite single lap joints with spew fillets under static tensile load were investigated numerically using the commercial finite element code ANSYS. The submodeling technique was employed to overcome the problem caused by extra aspect ratio of elements. The computation results indicated that the existence of spew fillets will not change the possible failure location, but will improve the anti-failure performance of single lap joint.