This research aimed to 1) develop science curriculum on environmental conservation, with an emphasis on the promotion of critical thinking skills for Mathayomsuksa 1 students, 2) compare students' learning achievement outcomes, basic science process skills, environmental conservation attitude and critical thinking skills before and after the application of this developed curriculum, and 3) compare the score differences in the areas of students' learning achievement outcomes, basic science process skills, environmental conservation attitude and critical thinking skills, after the use of this developed curriculum, in general and classified according to the differences in students' learning outcomes. The samples used in this research were 30 Mathayomsuksa 1 students, from Mathayomsuksa 1/3, Ban Wang Pikul school, Sam Pan district, Petchaboon province. They were selected by using Purposive Selection method. The research instruments employed were developed science curriculum on environmental conservation, wih an emphasis on the promotion of critical thinking skills for Mathayomsuksa 1 students, students' learning achievement test, basic science process skill test, environmental conservation attitude test and critical thinking test. The research results showed that the developed science curriculum on environmental conservation, with an emphasis on the promotion of critical thinking skills for Mathayomsuksa 1 students had the effectiveness index level as of 0.4114. Furthermore, Science Curriculum Development on Environmental Conservation, with an Emphasis on the Promotion of Critical Thinking Skills for Mathayomsuksa 1 Students 513 students had higher scores on learning achievement, environmental conservation attitude, and basic science process skill tests after the use of this curriculum, over all and in every aspect, at the statistically significant level of .05. Students with different learning outcomes also had higher scores on learning achievement, environmental conservation attitude, critical thinking and basc science process skill tests after the use of this curriculum, over all and in every aspect, at the statistically significant level of .05. Additionally, in general, the group of students with high scores had higher scores on learning achievement, environmental conservation attitude, critical thinking and basic science process skill tests than those with low scores at the statistically significant level of .05.