School principals play an important role in developing high-performing schools. To meet the challenges of the 21st century, they need quality preparation and professional learning before their assignments. However, there are significant challenges to find out high quality people who are physically, mentally, and emotionally ready to join preparation training course. This study aims to examine the effect of the Online Matriculation Program (OMaPro) on the readiness of prospective school principals for Principal Preparation Training (PPT). A pretest-posttest quasi-experimental design was used in the study, in which a total of 189 prospective school principals participated in the pretest and posttest of the first PPT cohort. 129 prospective school principals participated in both the pretest and posttest, with 99 students taking OMaPro before training, and with 30 who had never followed a matriculation program before training serving as the control group. The results of the study show that OMaPro positively influenced the cognitive, behavioral and emotive-attitude readiness of the participants. Unfortunately, the program cannot run in rural areas because of the lack of internet access. The results should be helpful for prospective school principals to prepare themselves before training, and also for education authorities and policy makers in order to maximize the training results and manage the training programs.