Based on the DMSP F13 Satellite observations from 1995 to 2005, the longitudinal distributions of the electron temperature (T(e)) and total ion density (N(i)) in the sunset equatorial topside ionosphere are examined. The results suggest that the longitudinal variations of both T(e) and N(i) exhibit obvious seasonal dependence as follows: (1) wavenumber-four longitudinal structure in equinox, (2) three peaks structure in June solstice, and (3) two peaks structure in December solstice. Moreover, the longitudinal variations of T(e) and N(i) show significant anti-correlation, and we speculate that the longitudinal variation of T(e) may result from that of N(i) which can control T(e) through the electron cooling rate. The wavenumber-four longitudinal structures of both T(e) and N(i) in equinox may relate to the eastward propagating zonal wavenumber-3 diurnal tide (DE3), which has effect on the amplitude of the daytime zonal electric field. The longitudinal variation of T(e) and N(i) in the two solstices may be caused both by longitudinal variation of geomagnetic declination and DE3.