Online formative assessment is still challenging although it is getting an increased attention as a significant tool for diagnosing and analysing students' motivation and achievement in various education domains. This research examines the effects of an online formative assessment tool (OFAT) about science motivation and achievement in second-year students' university education during four consecutive academic years, 2014 to 2018. A research on the basis of a randomized experimental design was conducted that assigned groups used an OFAT along with various assessments that students participated. A total of 311 students enrolled in the subject take part in the research, respectively 94, 89, 59 and 71 students. Particularly, the OFAT is offering feedback from students, feedback from lecturers and adaptive assignments. Here, data contained student motivation survey data, standardized achievement pre- and post-test data and students' log records. The results of multiple tier analyses exposed positive effects about students' motivation and achievement. Based on usage measurements, students' intensity offers the positive effects about students' motivation and achievement. Furthermore, along with overall students' improved performance, the effects of high-performing students' achievement were higher. Therefore, the results acquired meaningfully contribute to recover main drawbacks and difficulties of traditional science learning programs.