Korea has two structurally similar but phonetically different number systems being used simultaneously. One is somewhat irregular informal system, and the other regular formal system. This study is to examine the effect of Korean dual number system on primary school children's arithmetic ability by analysing the performances of counting, dictation, comparing, addition and subtraction in the two number systems. Subjects in this study were children ranging from 3 to 11 years old. Tasks were developed to measure the ability to understand and switch the two number systems in couning, dictation, comaring and calculation. The result of this study revealed that Korean children have difficulty in understanding the numerical value of informal numbers and feel confused in the comparing the two numer systems. Children showed even greater difficulty in converting informal numbers into formal ones in calculation. Therefore, it seems nesessary to pay more attention to the use of two different number systems in primary school arthmetic instructions.