Information about gaps and their sizes distribution is useful for protecting herbaceous diversity. This study was conducted to investigate the effect of gap size on herbaceous species diversity in oriental beech (Fagus orientalis Lipsky.) stand in Asalem, Guilan province. For this study, fifteen gaps in small, medium, and large sizes were randomly selected. Then, herbaceous species were identified in 4 m² micro plots within the gaps and percent covers of herbaceous species were estimated in the basis of Domin’s criterion. Diversity indices including Shannon-Wiener, Mc Arthur, Hill, species richness and Smith -Wilson, the modified Nee’s, the modified of Hill’s evenness indices were computed. The results revealed that with increasing gap size, the diversity and richness of herbaceous species have been significantly increased, while evenness indices showed no significant differences in different gaps. The highest evenness values were obtained in the small gaps. In small, medium and large gaps 22, 24 and 25 species were found, respectively.