The purpose of this study was to test the effectiveness of a courseware, $quot;Drosophila`s Genetics$quot;, and to analyze teachers` and students` opinions on the courseware. Drosophila`s Genetics is a computer simulation developed by the authors for teaching the Mendelian principle of genetics in the middle school. Nonequivalent pretest posttest control group design was used to test the effectiveness of the courseware which was used as a review lesson. According to one-way ANCOVA, the difference of learning achievements between the teacher lesson group (n=53, adjusted mean=7.8) and the courseware lesson group (n=30, adjusted mean=11.4) proved to be statistically significant (F=25.28, p=0.00). The analysis of teachers` and students` opinions about the courseware revealed that the courseware was not only useful to the improvement of understanding genetics and helpful to the development of scientific thinking ability but also exciting and wall organized for individualization. But their evaluation of the feedback given from the courseware to she students was that the feedback should provide more in formation.