This article presents an analysis of plant species richness and diversity, concerning some climatic variables along a 1500-m elevation gradient on the Binalood Mountain in Heydari Wildlife Refuge (HWR), northeastern Iran.  Two hundred and thirteen nested-sampling quadrats were established and the abundance of the plants was recorded. Vegetation sampling was carried out from 2014 till 2016, following the phytosociological procedure. The size of the main plots was 100 m² with subplots of 1 and 25 m² inside. Hill numbers were used to assess the species richness and diversity. The relationships between species richness, diversity and environmental variables, i.e., temperature and precipitation, were obtained by using simple linear regression models. HWR was studied in order to: (1) examine the significant diversity pattern existing along the elevational gradient; (2) determine the main climatic variables associated with this pattern; (3) to establish whether changing the width of elevation classes modified the pattern or not. The monotonic and reductional diversity pattern in the study area indicates the importance of performing conservation programs for the maintainance and improvement of floristic diversity on accessible lowlands of the HWR. Modifying the width of elevation classes did not alter the general trend of the reductional diversity pattern. The regression curves suggested that differences in plant diversity along elevational gradients may be mainly influenced by temperature.