Inhibition effects were investigated for new type of antiseptic natamycin and the combination of natamycin and sodium diacetate on common mildews, including Aspergillus niger, Rhizopus niveus, Mucor corticolus, Penicillium glaucum. The results indicated that the minimal inhibition concentration of natamycin was 3～6 mg/L, while the optimized inhibition ratio was 4 mg/L to 3 g/L in the combination. Destructive storage test was also conducted at 10, 20 and 30 mg/L respectively in this paper, and little difference were observed according to the result by plate counting. 100% inhibition effect of natamycin at 10 mg/L could be expected to last for 18 days in liquid pet feed and 24 days in liquid coral feed respectively by eyes observation, while 96.3% and 94.3% inhibition effect of natamycin were observed at the same concentration after 30 days by plate counting. According to the results, the only natamycin at the tested condition was not able to 100% inhibit fungi in feed within 30 days.