Some students especially those with particular deficiencies in arithmetic calculations are unable to correctly perform fundamental calculation operations (addition, subtraction, multiplication, and division). They frequently make calculation errors and give up easily. Researchers interested in this area have found the cause of these problems in the teachers insufficient and incorrect teaching or students inability to acquire fundamental concepts (one to one corresponding, sets, etc.). By analyzing the calculation errors of these students, we found that they show similar patterns in their errors. By understanding these patterns teachers can succeed in solving students problems and reduce their difficulties in written comprehension to a minimum. In order to achieve this goal, 20 students who attended the Center for Special Learning Difficulties in the city of Yassoj were examined, and after evaluation by the experts, they were diagnosed as discalculia students. The errors of the students in the operation subtest of the achievement test of Iran - key math analyzed and grouped. The results showed that one of the main difficulties of students with dyscalculia is recognizing the facts in mathematics. Other errors are the result of a problem in the comprehension of strategic knowledge, the time and place of using facts or definite techniques or are well the result of chance errors.