Abstract: Dual-energy computed tomography(DECT)can be used for computing atomic number and electron density.As a result,we can discriminate unknown materials.One approach to deal with DECT is by basis material decomposition method.However,this method leads to a large error and bad suppression of metal artifact.This paper focuses on methodic error analysis of basis material decomposition method.Reasons for bad suppression of metal artifact are explained.Also,this paper derived the theoretical formula of methodic error.We give the applicable conditions of this method.In general,this method leads to a small error for low-Z materials and a large error for high-Z materials.