The major objective of this paper is to empirically examine the effect of process maturity on the performance of industrial R&D projects. Process maturity, a fundamental concept of the Capability Maturity Model developed by Software Engineering Institute, represents the essential of Total Quality Management (TQM). Based on literature, our research model constructs process maturity in terms of structured process, goal setting and controlling, metrics, and process learning; and links it to the R&D performance that consists of technical, commercial and managerial successes. The model also includes firm size as a moderator of different effects that process maturity may have across firms. Measures for process maturity are based on the best practices identified in literature. Data are obtained from 77 successful R&D projects carried out by Korean manufacturing firms. Multiple regression and t-test are used to test proposed hypotheses. Findings are as follows. (1) In the R&D process, process maturity partially contributes to the performance of R&D projects. More specifically, goal setting and controlling-related practices drive both technical and commercial successes, while process learning-related practices drive commercial success. In contrast, traditionally emphasized elements such as structured process or metrics are found not to be significant. (2) The degree of process maturity is significantly higher in large firms. (3) Process maturity impacts on commercial success in the case of large firms, whereas it does on technical success in the case of small firms. The results imply that the TQM principles are partially associated with R&D performance, and the nature of benefit from high maturity could vary according to firm size.