Abstract: The major goal of science education today is fostering students intellectual competencies, such as independent learning, problem-solving, decision-making and critical thinking. In order to achieve this, science teaching must be shifted from traditional schooling to more constructivist-oriented instruction. The aim of this study was to find out the effects of various instructional practices such as problem based learning, 5 E model instruction and portfolio writing practices on enhancing the achievement of low achievers. Pre test- post test parallel group design was adopted for the present study. The sample consisted of 120 low achievers, where 90 students were categorised in three experimental groups and 30 students in control group. Instructional materials were prepared based on the topic from 8th standard science text book according to the nature of instructional strategies selected. The major finding of the study is that the select instructional strategies are more effective than the traditional method of teaching science. Among the instructional practices, the problem based learning practice is more effective in enhancing the achievement of low achievers.