Relationship between the slip line and the orientation of grains in polycrystalline copper during tensile plastic deformation is studied. Slip plane of each grain is calculated from the orientation obtained using EBSD technique, and the slip line of specimen surface is observed by SEM image. A method of estimation of active slip plane in each grain of polycrystalline copper during tensile plastic deformation is proposed based on the experimental results. Direction of slip line is calculated from the geometrical relationship between the specimen surface and the slip plane whose orientation is obtained using EBSD method. The estimated results and the observed results are compared by use of their correlation factor. The constraint between grains in the normal direction to the specimen surface is expected to be smaller than that between grains in the surface plane. Namely, the active slip plane is decided from the condition that the component of the specimen inward direction or the direction on the specimen surface of the shearing stress on the slip plane is maximum. It is also shown that the small size grain deforms on the slip plane whose orientation is close to the active slip plane of the neighboring grain with the maximum size. The estimated results are in good agreement with the experimental results.