RNA silencing is a highly conserved mechanism in many eukaryotic organisms that involves sequence-specific RNA degradation. One of the important roles of RNA silencing is antiviral infection. To counter this defense mechanism, many plant viruses have evolved or acquired functional proteins (suppressors) for suppression of RNA silencing. Geminiviruses are a family of small circular single-stranded DNA (ssDNA) viruses that cause severe diseases in major crop plants worldwide. AC2 (C2) and AC4 (C4) proteins of many geminiviruses were reported to be suppressors of RNA silencing. Ageratum yellow vein China virus (AYVCNV) and Stachytarpheta leaf curl virus are two distinct geminiviruses identified in China. In this study, the functions of C4 of AYVCNV and StaLCV were studied, analysis of infectivity of AYVCNV and StaLCV C4 genes using Tobacco rattle virus (TRV) vector were carried out in Nicotiana benthamiana plants, N. benthamiana plants expressing C4 can induce the virus-like symptoms, indicating that the C4 gene of AYVCNV or StaLCV is symptom determinant. In experiment of suppression of RNA silencing, C4 protein of AYVCNV or StaLCV can suppresses the local silencing of gfp in transgenic N. benthamiana plants (16c), suggests that both of them are suppressors of RNA silencing. The above results indicate that C4 of