This report is on the development, based upon mastery learning theory, of a system of instruction for elementary school level arithmetic classes which would best match the requirements of the educational situation. It is one of a series of studies made during the past several years. This report gives the results of applying this system to actual instruction in order to verify its effectiveness and show its feasibility. Four units in arithmetic from four elementary grade levels were selected for use in this study. The results are as follows : (1) According to summative testing results, for the lower grades 90% of the pupils belonging to the mastery learning groups achieved 90% mastery. (2) Remedial learning for low achievers (identified through arithmetic achievement and/or readiness tests) was improved significantly. (3) For the higher elementary grades, 80% of the students attained an 80% or higher level of mastery of the instructional objectives. (4) Overall results showed that the instructional materials developed for this study were significantly more effective than regular instruction.