Abstract: In view of the T-slotting defects on the outermost layer at the cut-out side on plain-woven carbon fiber reinforced plastics (CFRP), the cutting models on the outermost layer of fibers at the cut-out side were established.The slotting tests, using a T-slot cutter, were carried out to study the formation mechanism of the T-slotting defects.In the cutting process of the outermost layer at the cut-out side, there are Case A and Case B, for respectively the cutting of fill fibers with the support of warp fibers and the cutting of fill fibers without the support of warp fibers.The results show that when the feed per tooth increases, the residual factor of the outermost layer at the cut-out side basically decreases while the delamination factor increases.And residual factor mainly experiences 2 times of decreasing in succession, including the reducing of residual fill fibers of Case A and the reducing of residual warp fibers.Residual fill fibers of Case B are ubiquitous in each test, but some of them can be removed effectively when the feed per tooth region is appropriate, in which Case A fibers are removed.Using a fine grain cemented carbide cutter for slotting, the appropriate feed per tooth region is between 11.22 μm/z and 15.62 μm/z.The changing rules reflected by the theoretical derivations are in good agreement with the test results.The study has guiding significance for the selection of feed per tooth and the inhibition on the T-slotting defects.