Based on the lidar observations from April 2001 to December 2004 over Wuhan, the long term （time scale longer than one year） and nighttime variations of background Na layer are presented, and their possible causes are also discussed. The observation results of long term variation show that Na abundance has a maximum value in November, which is approximately 2 times of the minimum value （1.6 × 10^9 cm^-2） in May; Na centroid height reaches a maximum value in August, which is about 1.4 km higher than the yearly average value （91.2 km）; the average value of rms width is 4.5 km, and its maximum and minimum values occur in December and March, respectively; Both of the Monthly averaged centroid height and rms width show a quasi-semiannual variation, but with contrary phase variations. Except the long term variations, the background Na layer also exhibits obvious nighttime variation： Na abundance increase with the nighttime except a short duration near the dawn time with a maximum value of 2.9 ×10^9 cm^-2 at 0540 LT; the height of peak value decreases with the nighttime; the centroid height of Na layer increases slowly with the nighttime until the time of 0520 LT, then it decreases rapidly to the value at the time of 2000 LT; the magnitudes of the fluctuations for the averaged Na layer abundance, height of peak, centroid height and rms width in the nighttime are 1.2 × 10^9 cm^-2, 3km, 0.3km and 0.8km, respectively.
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