In situ photosynthetic nitrogen-use efficiency (PNUE, photosynthetic capacity per unit leaf nitrogen) was investigated in species that commonly distributed at different altitudes (600–3700 m above sea level) on Mount Kinabalu. Photosynthetic nitrogen-use efficiency was lower in species at higher altitudes, with a mean PNUE at 3700 m being one-third as large as that at 600 m. This difference in PNUE was larger than that explained by the biochemical response to lower air pressures only. Across altitudes a negative correlation between 13C abundance (δ13C) and PNUE was found. Species at higher altitudes tended to have higher δ13C, suggesting that they had a lower conductance for CO2 diffusion from the air to chloroplasts. The lower conductance might be responsible for the lower PNUE in species at higher altitudes. Although leaf nitrogen content per unit area tended to be higher at higher altitudes, it did not seem to contribute to increasing photosynthetic rates. Thus, the idea that a higher nitrogen content at higher altitudes is a compensation for a lower PNUE was not supported. In contrast to the large difference in PNUE among altitudes, PNUE tended to converge within a narrow range among species growing at the same altitude.