Tropical mountain forests are important reservoirs of biodiversity. They are usually species rich and often support endemic species making them prime targets for conservation effort. The aim of this study is to investigate elevational patterns of species diversity and phenology in order to provide a meaningful understanding of insects’ spatio-temporal distributions along a tropical gradient of elevation. Our study focuses on the Empidinae communities (Diptera, Empididae) from Doi Inthanon (north Thailand, gradient 400–2556 m.a.s.l), sampled during two entire years (2006–2007, 2014). This group of insects is more diverse in temperate localities than in the tropics and we found that (1) increase in altitude and latitude has a similar effect, so that the diversity of our model increases with elevation, (2) the phenology is strongly influenced by seasonality with a peak of diversity occurring during the transition between the dry and rainy seasons, (3) there is no phenological shift in the diversity peak with elevations, (4) the species composition changes along the altitudinal gradient and through the year (high beta diversity). The increase of diversity with increasing elevation and the peak of diversity occuring at the transitional period both strongly coincide with abiotic factors, the decreasing temperature and the arrival of the monsoon, respectively.