The activity of chitosan and oligo-chitosan on several plant insects and pathogens were studied. Chitosan was active against lepidopterous and homopterous insects; the mortality was 80%. The insecticidal activity of chitosan to Plutella xylostella L. was higher than that to Spodoptera exigua Hübner. The mortality of six types of aphids was generally 60–80%, and the highest was 99.7%. Some fungicidal activity by oligo-chitosan was observed on 12 plant pathogens. The inhibition effect of oligo-chitosan to plant pathogens was improved with increasing oligomer concentration. The highest inhibition rate (80%) was against Phomopsis asparagi (sacc.a) Bubak (95%) and the inhibition rate against Fusarium oxysporum (Schl.) f. sp. Cucumernum owen, Rhizoctonia solani Kuhn and Fusarium oxysporum Schl. F. Sp. Uasinfectum (Atk.) Snyd. & Hans.