In the injection molding of long fiber reinforced resins, the greatest challenge to realize high strength molded products is minimizing the breakage of fibers during the plastication process inside the heating cylinder and during flow inside the mold. In the previous report 1), a melting model of 50wt% long glass fiber reinforced PP (L-GFPP) was proposed based on the visualization analysis of the continuous plastication process. Injection molding reciprocating plasticization is an unsteady process in which the relative positions of the screw and cylinder intermittently and periodically fluctuate corresponding to the molding cycle. In this study, a full flight screw and 50wt% L-GFPP were used to investigate the correlation between the reciprocating plasticizing process of L-GFPP and fiber length distribution during this process. In the plasticizing process, the formation of temporary unfilled area and the rotational behavior pellets moving from the orthogonal orientation to the parallel direction relative to the flight were observed. The results also showed that solid-bed fragments tend to generate easily in the compression zone. In addition, it was confirmed that the solid-bed ratio decreases temporarily in the middle of the plasticizing process and this is influenced by screw revolution and charge stroke. The weight average fiber length in the injected resin showed a temporary decreasing tendency in the middle stage of the injection process. Reference (1) S.Ma, H.Yokoi; Seikei-Kakou Symposia’13, 73(2013)