Managing multiple projects seem to be the current trend in project management. This type of multiple-project management has become common in many industries, particularly in Oil & Gas. It is not easy for one project manager to manage multiple projects simultaneously and effectively. Unfortunately only few researches done to determine factors affecting Management of Multiple Projects effectiveness. This study is a continuation of previous research that identified major key drivers of effectiveness in Managing a Group of Multiple Projects (MGMP), and it recommends to research Teamwork Quality (TWQ) as another key driver of effectiveness. The objective of this research is to investigate the impact of TWQ on the effectiveness of MGMP in Oil & Gas companies. TWQ is measured against: Communication, Coordination, Balance of member contribution, Mutual support, Effort and Cohesion. Whereas, Effectiveness is measured in terms of project’s performance (i.e. schedule, budget and customer satisfaction), and learning (i.e. acquisition of knowledge, skills and expertise). The research conducted using a survey of approximately 40 multiple-project manager from 8 major Oil & Gas Companies. Total of 196 team members, team leaders and managers will participate in this survey. The preliminary findings show that TWQ has positive impact on project’s performance and learning.