In 1984 Benjamin Bloom stated that one-to-one tutorial support increased students' achievement by two standard deviations (two sigma problem). In the literature for solving two sigma problem (in order to increase student achievement), many systems were developed as intelligent tutoring systems, adaptive hypermedia systems, recommender systems and etc. In addition to this, it can be said that dynamic assessment increases the students' achievement. Because one of the main objectives of the dynamic assessment is to improve the performance of learners by providing instructional aids with assessment tasks. Within the scope of this research, a web-based dynamic assessment system was designed and developed. Then, the effectiveness of the system tested with actual users. One group pretest-posttest design was used. A total of 322 secondary school 5th-grade students participated in this study. The findings were discussed in the context of two sigma problems. According to the findings, it can be said that the web-based dynamic assessment system increases student achievement approximately two standard deviations. [For the complete proceedings, see ED608557.]