The aim of the study is to investigate the effects of using computer for developing arithmetical skills of students with mathematics learning difficulty (MLD). The study was carried out with pre-test - post-test single subject research design. The participants of the study consist of a girl and two boys who attend 3rd grade at elementary school. The contents of the computer-aided instruction materials consist of counting skills, place value and addition subjects which are related to 1st and 2nd grade mathematics course learning outcomes of primary school. The materials have been prepared in the light of educational neuroscience findings about mathematical cognition. Participants were given a total of 75 lessons of individual instruction for five weeks, every day on weekdays and 20-30 minutes a day with the materials developed. Achievement tests developed by the researcher were used as data collection tools. Test response times and correct answers of the participants were identified in the achievement tests. It was found that progress was achieved in counting skills and place value concepts and they could solve addition problems. However, it was revealed that the difficulties of students at the second grade level continued with addition, especially when the numbers were given side-by-side. It can be stated that computer assisted instruction used in this study developed students' arithmetical skills and increased their speed. Triple code and the models which help reduce the workload of working memory are advised to be used in the instruction of the students having MLD.