The object of this study is to investigate the dose-response characteristics of an amorphous silicon electronic portal imaging device (a-Si EPID). An in-housing developed DVS(Dosimetric verifying system) software was used to analysis the portal image captured in different conditions, and the relationship of pixel value with dose, field size, dose rate, readout signal variation in different binning mode, gain mode and integration time setting was investigated. The EPID showed an excellent temporal stability on short time .Ghosting effects could increase the sensitivity of the EPID and became more pronounced with decrement of time intervals between two exposures as well as with increment of dose though the EPID response is linear with dose. However, it will be ideal for dosimetry application if some corrections for scatter could be made.Cited