This study aims to assess the impact of hypermedia designing as a cognitive tool on knowledge acquisition and retention in comparison to traditional instruction. A pretest-posttest control group design was used. The study included 48 second-year preservice computer teachers who enrolled in "Instructional Technology and Material Preparation" course at one of the universities in Turkey. The subjects were assigned to experimental and control groups through "matched-pair technique." Both groups were given a pretest to measure their prior achievement level in "Instructional Technology & Material Preparation" course content. While the control group was exposed to traditional instruction, the experimental group developed hypermedia on the course content in a constructivist learning context for 14 weeks. A posttest was given at the end of the treatment, and a retention test was given eight weeks after the treatment to both groups. The results showed that the use of hypermedia as a cognitive tool resulted in similar level of student achievement as traditional instruction.