Cucumber mosaic virus (CMV) and Pepper mild mot- tle virus (PMMoV) causes severe economic loss in crop productivity of both agriculture and horticulture crops in Korea. The previous surveys showed that naturally available biopolymer material – chitosan (CS), which is from shrimp cells, reduced CMV accumulation on pepper. To improve the antiviral activity of CS, it was synthesized to form phosphate cross-linked chitosan (PCS) and compared with the original CS. Initially, the activity of CS and PCS (0.01%, 0.05%, and 0.1% con- centration) compound against PMMoV infection and replication was tested using a half-leaf assay on Nico- tiana glutinosa leaves. The total number of local lesions represented on a leaf of N. glutinosa were counted and analyzed with phosphate buffer treated leaves as a neg- ative control. The leaves treated with a 0.1% concentra- tion of CS or PCS compounds exhibited an inhibition effect by 40-75% compared with the control leaves. The same treatment significantly reduced about 40% CMV accumulation measured by double antibody sandwich enzyme-linked immunosorbent assay and increased the relative expression levels of the NPR1, PR-1, cysteine protease inhibitor gene, LOX, PAL, SRC2, CRF3 and ERF4 genes analyzed by quantitative reverse transcrip- tase-polymerase chain reaction, in chili pepper plants.