The larvae of Liriomyza sativae Blanchard leafmining activity and its instar distinguishing were studied in this paper. The results indicated that, 1) larval instar could be distinguished from the length of cephalopharyngeal skeleton; 2) the mean mine lengths mined by the larvae showed significantly differences under various temperatures treated , no significant differences were observed on its mean mine area; 3) both mean mine length and mean mine area mined by the larvae were changable with their host plants; 4) the third instar larvae was the main one of larvae to damage, its mean mine area occupied over 82% of total larval infestation, and 5)using the mine length mined by the larvae could conveniently determine the instar sizes. The relationships between the mine length mined and instar sizes at various common host plants were described in the paper.