The main objective of this study is to find the relationship between capital structure, free cash flow, and performance of oil and gas companies listed on PSX over the last eleven years (2005-2015). The study is quantitative in nature, follow positivistic paradigm and deductive approach. The study employ capital structure measure, total debt to total assets ratio, and free cash flow as the independent variables of the study, while firm?s performance measures, return on assets and Tobin?s Q are included as the dependent variables of the study. The study also employs firm size, assets growth, interest rate, inflation rate and world oil prices as the control variables. The population of the study consists of all oil and gas firms listed on PSX and financial period under consideration is from 2005 to 2015. The whole oil and gas sector consisting of twelve companies is under study but due to unavailability of financial data of two companies for full period under consideration, the final sample consist of ten oil and gas companies listed on PSX. Out of ten PSX listed oil and gas companies, four companies are involved in oil and gas exploration and production and the other six are involved in oil and gas marketing and distribution. Thus, to get better insight into the performance of both category of firms, overall PSX listed oil and gas firms were broadly categorized in two categories i.e. exploration and production companies and marketing and distribution companies. The required financial data is extracted from the annual reports of the sample companies and different statistical analysis is performed with the help of Stata version 13.0, to fulfill the objectives of the study. After assumption testing through Hausman Test, and BPLM Test, an OLS regression model is selected as the final model and interpretations are made accordingly. The study find a negative relationship between total debt to total assets ratio and firm?s performance measured through return on assets and Tobin?s Q for oil and gas exploration and production companies as well as for oil and gas marketing and distribution companies, suggesting that higher level of debt will decrease firm?s performance. The results also showed a significant positive relationship between free cash flow and both firm?s performance measures under study for oil and gas exploration and production companies as well as for oil and gas marketing and distribution companies, indicating a lack of evidence supporting the free cash flow hypothesis. The results also showed a significant positive relationship between assets growth and both firm?s performance measures under study for both categories of oil and gas companies, whereas, a significant negative relationship is observed between firm size, interest rate, inflation rate, world oil prices for both categories of oil and gas companies under study and both firm?s performance measures.