The daily variation of ion temperature observed with the European Incoherent Scatter (EISCAT) Troms¿ UHF radar in the upper F region (from 300 to 500 km) has been found to be significantly dependent on season. Of particular interest is the statistics that the ion temperature at local night is higher than that at local daytime for near solar maximum summer conditions; by contrast no differences for winter. Using the ion energy equation estimates of the heat exchange with electrons suggest these local time and seasonal differences. A case study using summer-time data from the Sondrestrom incoherent-scatter radar (ISR) also shows local-time dependences similar to that observed with the Troms¿ ISR. The case study using a summer-time data from the EISCAT Svalbard radar (ESR) shows the opposite sense. These seasonal characteristics of the ion temperature that depend on the observation site suggest that the day-night difference of ion temperature in the polar upper F region has complicated magnetic-latitudinal dependences.