This study explores the optimal capacity size and it`s impacts from the provider side of tourism resources interpretation. Data were collected from the in-depth interviews with seven cultural tourism interpreters of three national heritage tourism sites in Korea. The results show that the interpreter`s explication behavior was collapse of the interpretation contents, shortening of interpretation time, one-sided interpretation progress. Also, it had appeared a negative impact to the noise-induced, a lack of leadership, occurrence of missing children, and optimal interpretation of evidence from this study group of size 15 patients was enough. The interpreter`s and tourists` eye contact while the size group of about 15 of the interpretation was to identify the size of the interaction, or can do enough interpretation. In the case of children group, the effective interpretation size was 6 to 8 children. The findings imply that the tourism resources interpretation is urgently needed a new approach and system to improve interpretation quality and communication between tourists and interpreters based on the optimal capacity size of interpretation.