Field experiment was conducted to investigate the effects of K supply on diurnal variations of net photosynthetic rate, transpiration rate, stomatal conductivity, intercellular CO2 concentration and water use efficiency in barley leaves at heading stage under enhanced UV-B radiation at the Station of Agricultural Meteorology, Nanjing University of Information Science and Technology, Nanjing, China. The experiment was designed with two UV-B radiation levels, i.e. ambient (CK, 1.5 kJ/(m2·h)) and enhanced by 120% (1.8 kJ/(m2·h)); and two K supply levels, i.e. 73 kg K2O/hm2(low) and 150 kg K2O/ hm2(high). The results showed that enhanced UV-B radiation declined chlorophyll content, net photosynthetic rate, transpiration rate and stomatal conductivity in barley leaves, but had no obvious effect on intercellular CO2 concentration and water use efficiency. Increasing K supply could alleviate the damage of enhanced UV-B radiation to net photosynthetic rate from enhanced UV-B radiation, but not to transpiration rate and stomatal conductivity.