Abstract The main purpose of this research was to find out the contributions of decomposing method in addition and subtractions of two digit numbers, the respondents for this research were second grade students from Ahmed guray elementary school in Jijiga. To know the contributions of decomposing method there were questions raised to use it to answer the decomposing method contributions on addition and subtractions of two digit numbers to get the necessary data we have applied a mix method approach we have prepared a pretest and a posttest to get the quantitative data and we have also prepared an interviews with some of the students to get the qualitative data. After analyzing the data using SPSS independent sample t-test and compared the pretest of the control group and the treatment group we have come up with the result that there is no significant different between the control group and the treatment when considering the items or indicators number relations, properties of addition and subtraction and justifying the answers, but when it comes to the analyzes of the post-test there is a significant different between the control group and the treatment group when we used the indicators number relations, properties of addition and subtractions and justifying the answers, which means that the decomposing method does have more significant impact or decomposing method does contribute to the addition and subtractions of two digit numbers of second grade students.