The purpose of this study is to investigate and compare the effects of activity-based and traditional instructions on students' conceptual development of certain probability concepts. The study was conducted using a pretest-posttest control group design with 80 seventh graders. A developed "Conceptual Development Test" comprising 12 open-ended questions was administered on both groups of students before and after the intervention. The data were analysed using analysis of covariance, with the pretest as covariate. The results revealed that activity-based instruction (ABI) outperformed the traditional counterpart in the development of probability concepts. Furthermore, ABI was found to contribute students' conceptual development of the concept of "Probability of an Event" the most, whereas to the concept of "Sample Space" the least. As a consequence, it can be deduced that the designed instructional process was effective in the instruction of probability concepts. (Contains 6 tables and 5 figures.)