The aim of this study is to investigate the effects of Cost Accounting Techniques on financial performance of both large firms and small & medium-sized enterprises (SMEs) in Egypt. This will be accomplished by analyzing the influence of marginal cost and absorption cost on both return on assets and net profit margin, and by tracing if there are any differences between the impacts on small and medium enterprises and large firms or not. An empirical study was conducted on both small and medium enterprises and large firms, and after collecting questionnaire results from those who work in small and medium enterprises and large firms, results from four hundred valid questionnaires for both SME's and large firms were thoroughly analyzed. The findings suggest that there is a direct impact of marginal cost and Absorption cost on both return on assets and net profit margin for both small and medium enterprises and large firms.