The mechanical in-plane shear testing method for advanced composite panels has not yet been authorized internationally because of its difficulty to present the uniform shear stress field and the reliable shear properties. In the present paper, the several revisions of the traditional picture-frame loading fixture are proposed to use for the in-plane shear testing method. The experiments on unidirectional and cloth-reinforced CFRP specimens and the numerical analyses by the use of FEM were performed in order to verify the advantages of these various revisions. As a result, the uniform pure shear stress state and hence the reliable higher shear strengths were obtained, because there is no large stress concentrations along edges and at corners, compared with other testing methods which used the traditional simple picture-frame fixture, two-rail or three-rail shear fixtures and JIS method. That is, in these shear tests which are used most widely at present, the lower shear strengths were obtained because of the premature failure due to stress concentration and nonuniform stress distribution. Further, it seemed that JIS method for GFRP did not give a shear fracture under combined stress conditions in unidirectional composites.