An on-line automatic resin pellet external testing and automatic selecting control system for substandard products has been developed. The system utilizes the advantage of image processing technology and the testing targets are as small as 50 similar to 100 mu m foreign substances and discoloration substances in the pellets. The testing system includes pellets sampling from a plant, testing of sub-standard pellets, and subsequent disposal of tested pellets, and is fully automatic. The resin pellets are transported automatically to product silo or sub-standard product silo by the selecting control system based on the external testing result. The systems contribute to the stability of quality control and improvement of operation and labor saving.