The purpose of this study is to clarify a mechanism of the flow mark that appears on the surface of thin PC/ABS blend injection moldings through the observation of the internal structure at various processing conditions. The flow mark had two different constitutions, such as a luster part and a cloud part, alternately on the both surfaces. It was obvious that the luster part was a PC-rich state, and both the PC and ABS coexisted at the cloud part from the surface analysis of the article. The internal structure of the flow mark article was quite different from no flow mark article. In the case of no flow mark article, a clear fountain flow could be seen at the flow front, and the ABS could not be seen at the tip of the flow front from the observation, whereas in the case of the flow mark article, the center of the flow front went towards the mold thickness direction. As a result, the PC phase, which covered at the tip of the flow front, might be broken, and both PC and ABS coexisted, and thus formed the flow mark. Moreover, in the case of the flow mark article, it was found that the PC and ABS had already flowed with winding while they entered the mold cavity from the gate. Therefore, a designing for the gate may be most important to control the flow mark.