The purpose of this study was to investigate the effects of a Computer-Assisted Instruction Material (CAIM) related to "photosynthesis" topic on student cognitive development, misconceptions and attitudes. The study conducted in 2002-2003 academic year and was carried out in two different classes taught by the same teacher, in which there were fifty two 11th grade high school students, in central city of Trabzon in Turkey. An experimental research design including the photosynthesis achievement test (PAT), the photosynthesis concept test (PCT) and science attitude scale (SAS) was applied at the beginning and at the end of the research as pre-test and post-test. After the treatment, general achievement in PAT increased by 10% in favour of experiment group (EG) at (p<0.05) significant level. Although the increase in cognitive development at knowledge level was 14.8% in the EG and 18.2% in the control group (CG), the development at comprehension and application levels were 19.8-18.5 in the EG and 1.75-0.86 in the CG, respectively. This result showed that using CAIM in teaching photosynthesis topic was very effective for students to reach comprehension and application levels of cognitive domain. However, CAIM did not change major misconceptions related to photosynthesis topic in EG as expected. Meanwhile, same misconceptions in EG about source of energy for plants and their nutrition were decreased more than CG. It was also found out that there was little change about students' attitudes towards science education in both groups.