We studied flowering and fruiting seasonality of understory shrubs of genus Medinilla (Melastomataceae) in the lower montane forest on Mount Kinabalu, Borneo, in relation to seasonal change in air temperature, rainfall, and solar radiation. Eight sympatric species showed either annual or biannual patterns. The number of species bearing flower buds and flowers increased biannually, indicating that two seasons were favorable for flower budding and flowering of Medinilla species. Individuals of most species started flower budding during January-March, showing strongly negative correlations with seasonal patterns of mean or minimum air temperature in previous a few weeks. The number of fruiting species peaked during May-August and increased when air temperature and solar radiation were high. These findings suggested that flowering activity of most Medinilla species may be regulated by seasonality of air temperature through floral initiation stimulated by low temperature. In addition, the high fruiting activity during period with high temperature and insolation indicated that such climatic conditions may facilitate the development of fruits and seeds of understory plants growing in cool and poor-light conditions, because photosynthesis and other physiological reactions are activated to produce compounds necessary for the development of fruits and seeds.