The purpose of this study was to investigate the effectiveness of teacher training for critical-thinking instruction via a computer simulation--"The Computer Simulation for Teaching Critical Thinking (CS-TCT)." Seventy-five preservice teachers enrolled in the University of Virginia participated this study. The employed instruments were CS-TCT and The Questionnaire of Professional Knowledge about Teaching Critical Thinking. The applied statistical analyses included descriptive analyses, MANCOVAs, and ANCOVAs. The CS-TCT training resulted in improving the presevice teachers' overall effectiveness in critical-thinking instruction, in increasing their professional knowledge about teaching critical thinking, and in enhancing their use of positive teaching behavior during critical-thinking instruction. More specifically, the training yielded an improvement in the preservice teachers' content knowledge about teaching critical thinking and an increase in their use of teaching behavior related to improving both critical-thinking dispositions and skills in their students. While the preservice teachers significantly increased their content knowledge about teaching critical thinking, they, however, did not significantly increase their pedagogical content knowledge about teaching critical thinking.