Lactobacillus casei AST18 was screened as an antifungal lactic acid bacteria which we have reported before. In this research, the antifungal properties of cell-free culture filtrate (CCF) from L. casei AST18 were detected, and the antifungal compounds of CCF were prepared by ultrafiltration, and semi-preparative HPLC, and then determined by GC-MS. CCF was sensitive to pH and heat treatment but it was not affected by the treatment of trypsin and pepsin. Through the treatment of ultrafiltration and semi-preparative HPLC there were two parts of CCF which showed antifungal activities: part 1 and part 4. Lactic acid was identified as the main antifungal compound in part 1. In part 4, three small molecular substances were detected with GC-MS. The three potential antifungal substances were cyclo-(Leu-Pro), 2,6-diphenyl-piperidine, and 5,10-diethoxy-2,3,7,8-tetrahydro-1H,6H-dipyrrolo[1,2-a;1',2'-d]pyrazine. The antifungal activity of L. casei AST18 was a synergistic effect of lactic acid and cyclopeptides.