The research method of this paper was document Analysis, based on The psychology of Mathematics and Study Technology, to attempt to find the major source of errors in arithmetic and we supported our theory by several papers relating to four fundamental operations of arithmetic.Finally,we used the conclusions from the paper to examine the context of arithmetic of first two grades of Grade 1-9 Curriculum and gave some suggestion.In short,the main result of this paper is:“The main source of computational errors comes from misuderstanding and the source of misuderstanding comes from misuderstood words or symbols(MUs for short) and too steep a gradient(lack of groundwork),hence,MUs was one of the main source of computational errors.”This paper would discuss the influence of errors due to MUs(focused on computation).