The diurnal cycle of precipitation in the New Guinean region is studied on the basis of satellite observations from Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) measurements and regional atmospheric model simulations. The study focuses on the effects of both the land-sea breeze and the orographic forcing on the diurnal evolution of precipitation during the rainy season (January--March) in the region. The 7-year TRMM PR data composite reveals several distinct features of the precipitation diurnal cycle in the region. Precipitation bands develop in the inland coastal region in the late morning to early afternoon and migrate inland from both northeast and southwest sides of the New Guinean Island following the inland penetration of the sea-breeze fronts. A separate convective rainband develops over the central mountain ridge in the early afternoon as a result of the development of the upslope winds due to the elevated surface warming over the mountain in the morning hours. This mountain ridge rainband intensifies and becomes the dominant rainband as the coastal rainbands associated with the sea-breeze fronts weaken during the late afternoon and the early evening. In the midnight to the early morning the rainband over the mountaintop weakens as downslope winds develop and splits into two rainbands, propagating away from the mountain ridge, one to the north and one to the south, and weakens over the lowland some distance away from the coasts. Meanwhile a coastal rainband develops offshore on each side of the island in the late evening to midnight and remains strong through early morning before it migrates offshore. As a result, the rainfall rate peaks in the late afternoon to early evening in most land areas except for in the lowland regions between the coastlines and the mountain where the rainfall rate peaks during the midnight, while the rainfall rate peaks in the late evening to early morning in most coastal regions offshore. The distribution of the diurnal amplitude shows two maxima: one over the mountains and the other in the coastal regions offshore. Convective rainfall rate peaks in the late afternoon while stratiform rainfall rate peaks in the midnight to early morning. The latter dominates the large diurnal amplitude over the mountain areas in the early morning. The above broad features are simulated reasonably well in a control experiment with a high-resolution regional atmospheric model. A sensitivity experiment with the terrain removed is conducted to elucidate the role of orographic forcing in the diurnal evolution of both the local circulation and rainfall patterns. The results show that the orographic forcing affects the diurnal precipitation through three major processes. First, the orography increases the moisture convergence at low levels by blocking and deflecting the mean flow. Second, the upslope winds help initiate convection in the afternoon at the mountaintop. Finally, the deep convection over the mountain acts as a source of propagating gravity waves, which help initiate rainbands in the coastal regions offshore in the late evening to early morning. Implication of the results is discussed.