The purpose of this study was to examine the effects of PCK designed instruction on academic achievement and self-efficacy for underachievers in science of elementary school. The hypotheses were as follows: First, PCK designed science instruction would improve academic achievement of the children. Second, PCK designed science instruction would improve academic self- efficacy of the children. The subject was 14 students of forth graders of elementary school who are underachievers in science class. For group composition, self efficacy test and science achievement test were conducted and they were divided into experimental group and control group. Each group has 7 participants. A total of 16 sessions were conducted for 8 weeks, twice per week, with experimental group were taught PCK designed classes while control group were taught teacher-led explanation type class. The result of statistical analyses were as follows: First, the experimental group in PCK designed class showed more improvement on academic achievement than the control group in the teacher-led explanation type class. Second, the experimental group in PCK designed class showed more improvement on self efficacy than the control group in the teacher-led explanation type class.