The purpose of this study was to examine the effects of constructivist instruction on the fourth grade preservice science teachers' achievement, attitude towards science teaching of in Science Teaching Methods II course. Two groups were assigned from Hacettepe University Faculty of Education Department of Science Education. Experimental group consisted of 53 and the control group consisted of 50 preservice science teachers. Quasi experimental research design was used. Constructivist instruction was used in experimental group and traditional instruction was used in control group This research study was conducted in fall semester of the 2007-2008 academic year and lasted 15 weeks. A mixed between within ANOVA with repeated measures was used as a statistical technique for analyzing data. Statistical mean difference was obtained for all tests in favor of experimental group at the end.