Pepper powdery mildew causes increasingly economical damage due to increased cultivation of pepper in greenhouses. To assess resistance of pepper resources against pepper powdery mildew, we developed a novel evaluation formula for pepper resistance against powdery mildew. The evaluation formula named S index is as follows; (number of the highest disease leaf/top leaf number)-(number of the lowest disease leaf/top leaf number). Positive correlation (81%, P = 0.01) between S index and authentic disease leaf rate was observed from the pepper plants infected by powdery mildew. Various pepper species from our genetic resources were evaluated to identify pepper varieties conferring resistance against powdery mild using S-index. Capsicum frutescens accessions 3CA131 and C. baccatum accessions 3CA0162, 3CA174, 3CA176 showed high resistance to powdery mildew, but none of C. annuum was resistant. Results suggest that S-index proposed in this study is useful to assess resistance evaluation of powdery mildew in chili pepper breeding.