The purpose of this study is to evaluate the impact of Instructional Scaffolding (IS) on an online Social Collaborative Learning (SCL) environment upon engineering students' knowledge construction (KC) level. In addition., this study also investigate on how the IS cognitively steers engineering students towards KC and helps them reach a higher level of KC. This study then proposed a KC model in an online SCL environment integrated with IS that could nurture engineering students' knowledge construction level. A questionnaire, achievement test, posting scripts from Facebook discussions, and structured interviews were used for data collection. The methodology comprised two designs: a quasi-experimental for the quantitative approach, and a case study for the qualitative approach. The quasi-experimental involved the pre and post-test to be taken by 74 participants from one polytechnic in northern Malaysia to identify the improvement in their knowledge construction level. Meantime, the case study involved a process in providing the detail and depth of exploration in a real situation by obtaining the perceptions and perspectives of 10 engineering students. Content analysis and thematic analysis were used to identify the relationships between codes, themes, and between different levels of themes. A t-test indicated a significant increase in the mean score of the post-test in both of the learning environments, that is, the conventional collaborative learning (CCL) and the SCL environment supported by instructional scaffolding. Nevertheless, the engineering students in the SCL environment showed a significantly higher mean score if compared with those in the CCL environment (pre-test score; 3.05 vs post test score; 13.98). Simultaneously, comparing the combination of results in the percentage of knowledge construction level reveals that engineering students in the control group and in the experimental group demonstrated an increase for each level of knowledge construction whether they were in the CCL or in the SCL environment They illustrated different percentages for scores of argumentative knowledge construction (such as CCL=84.21 , SCL=86.11) and metacognitive knowledge construction (CCL=I3.16, SCL=64.00) between control and experimental group. Through content analysis, eight answer themes that affect engineering students' knowledge construction were identified. Nine answer themes also were identified regarding on how SCL characteristics supported by IS enabled engineering students to reach a higher level of knowledge construction. Based on all these findings, the researcher then produced a holistic knowledge construction model. It comprised the 8 essential elements of impact factors, such as students' cognitive pre-engagement, motivation, engagement and enhancement, explanation and guide, encouragement and praise, determination., comfort and engagement, as well as ease ofthe learning process in the instructional scaffolding strategy model. As a result, it is concluded that IS plays a vital role in the knowledge construction processes in order to help engineering students' construct their knowledge and reach a higher level ofthinking
