This report is based on the writer's Diploma Thesis presented in April, 1961, at the Manchester University Department of Educational Psychology. It was concluded from the research that within the testing material devised for the 100 junior-3 English children, aged 9 to 10, there were at least 5 main sources determining the difficulty in arithmetic items. These were, in the order of their magnitude, kind of arithmetical rules (O), number of terms (N), mechanical vs. problem arithmetic (M), introduction of unit of measure (U), and size of numbers (S). Other results obtained from the study are discussed in detail.