The aim of this research is to test the effect of different project complexities and project management competence of project managers in public sector mega engineering projects of Pakistan and their effect on project performance. This research study is applied in Pakistan. Complexities in a project are inevitable. These cannot be controlled or eliminated but one can have their better understanding to counter them resulting in better project performance.  Quantitative research techniques have been applied for this particular research study. A total of 100 questionnaires were handed out among respondents, of which 82 survey questionnaires were useful for further analysis. Hypotheses have been developed to test the effect of variables of  project management competence on project performance and subsequently the effect of Technical, Organizational and environmental complexity on project performance.   Results show that project management competence have a positive effect on project performance, like leadership, management skills, communication, ethics, honesty have a strong impact on project performance. Secondly, Technical and organizational complexity have a negative effect on project performance whereas hypothesis of environmental complexity has not supported the project performance.  Further, a number of complexities were high ranked. For instance, goals, tasks and market conditions were ranked very high with respect to their impact on project performance.  This study contributes theoretically in the field of project management and on practical level study will not only help the Practioners in the field of PM, but also helpful for project managers of mega engineering projects. On societal level this study may help the Government intuitions in formulating rules and regulations such as PM certifications that can improve the project performance.