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