This paper tried to study the patterns of plant species diversity in a herbaceous vegetation under mowing management in southwestern Japan. The investigation was conducted to examine species response from community-level mowing to assess species diversity and richness in two large quadrats. Within the two quadrats, six 4 m2 plots each were diligently investigated and 110 vascular plant species were recorded. Analysis on diversity revealed that total number of species increased from 2001. There was an increase in rare and newly appeared species recorded after mowing (ANOVA p<0.01). After 2001, species diversity increased in both Q1 and Q2 of each year but a high diversity was seen in Q2 of 2003 and 2004 (p<0.05). It was also seen that the increase in species diversity and richness was determined by to biomass composition and invasion of new species after mowing. A high biomass level saw a decrease in species richness. Using DCA ordination, each plot was classified to determine the mean species richness which showed an increase from 2003 to 2004. It is from this finding that mowing is seen to sustain species diversity and richness and should be continued in order to conserve the unique biodiversity of Mt. Sanbe. These results suggest that management with mowing is an effective method although it reduced above-ground biomass, and periodic mowing proved to be successful in biodiversity conservation.