Capital structure composition is the financing decision which forms a critical aspect for any business entity. It involves making decisions on optimal mix of debt and equity to finance investment projects of a firm. Managers consider capital mix to finance operations and investments with the objective of increasing the financial performance of the firm. Petroleum firms are capital intensive in operations and investments, majority of petroleum firms in Kenya have closed their business like BP and Caltex but at the same time other firms have been started and expanded their business like Total Kenya, Shell and KenolKobil. The objectives of the study were to determine the effect of total debt on financial performance and to assess moderation role of firm growth rate on capital structure composition and financial performance. The pecking order theory as well as trade off theory were used to direct and predict impact of predictor variable on the outcome variable. The study adopted a descriptive comparative study design. The target population of the study was listed petroleum firms under the energy and petroleum sector of the Nairobi securities exchange. A comparative analysis of total Kenya and KenolKobil was conducted for the period of eleven years from 2007 to 2017. The study utilized secondary data collected from published financial reports. The study employed descriptive statistics and inferential statistics. Moderation effects were estimated using hierarchical regression and probed using SPSS PROCESS macro, model one. The findings indicated that total debt has a significant negative impact on financial performance measured by ROA and ROE. Additionally, the study revealed that firm growth rate had a significant moderating role on the relationship between total debt and financial performance. Further, it was revealed that listed petroleum firms in Kenya followed both the pecking order and the tradeoff models in their capital structure composition.