The problem based learning (PBL) is very interesting form of pedagogy based on present human learning cognitive theories. Constructivist theory of learning is the basis for problem based learning technique. Students have the central place in this learning technique, while, the teacher or educator works as guide or facilitator in learning process. Students’ work in small teams, face the problematic situation, hypothesize the situation, acquire new information, analyze it and finally reach at some decision by providing findings. This research study established to examine the effect of problem based learning technique on students’ academic achievement in subject of science at elementary level. An experiment was conducted with students’ of eighth class studying science subject at Government high school people’s colony, Gujranwala in session 2013-2014. Problem scenarios/ statements were framed out by the researcher based on two selected chapters of Text Book of Science of 8th class. Pretest, posttest control group design was used in the study. Seventy students out of 211 students were randomly selected as a sample of study. Further, the selected students were randomly assigned into two equal traditional and experimental groups. The students of experimental group worked in seven sub groups on problematic situations/ scenarios in their class over eight week period. The students’ of control group studied science as usual in their class by the class teacher in traditional way. Pretest was used on both traditional and experimental groups before starting the experiment. The purpose of using this test was to know the achievement level of both groups so that the researcher may able to compare achievement level after experimentation. Pretest and posttest were same and comprised of forty two multiplechoice questions (MCQs). The Achievement Test of Science (ATS) was consisted upon knowledge, understanding and application components of Bloom Taxonomy. The experimental group students’ were given Problem Based Feedback Form (PBLFF) after posttest to evaluate their opinion about problem based learning technique. After four months of period again, the students of both traditional and experimental group were given the achievement test of science. The purpose of using test is to evaluate their retention level and to examine, which group retained more knowledge. On the basis of acquired and analyzed data, the null hypotheses were tested. Independent samples t-test was used to establish the significant difference between mean scores of traditional and experimental group students on achievement test. Chisquare test was applied for the analysis of experimental group students’ opinion on PBLFF. The study results exposed that the experimental group students’ performed in a better way than control group students except knowledge domain, whereas, control group students performed slightly better than experimental group students. The experimental group students’ showed likeness regarding PBL technique as compared to traditional method of teaching. The results of retention test exposed that the experimental group students’ who worked in groups and responsible for their learning retained more knowledge as compare to traditional group students. On the basis of results, it was recommended that new mode of instruction like PBL be prescribed in the school teaching.