In a complex acoustical environment, the ability to discriminate the level of a sound is an important function of auditory system to accurately process sound signal information. Previous studies regarding human sound level discrimination were investigated in monaural condition. However, in natural acoustical environments, human discriminate sound level and spatial information binaurally. The effect of a preceding sound was determined upon the level discrimination of a successive sound by measuring the just noticeable difference (JND) of the level of the successive sound in binaural conditions. The data were collected in close-field, dichotic acoustical conditions. The level and the spatial azimuth of both the preceding and the successive sounds were manipulated by changing the average binaural level (ABL) and the interaural level difference (ILD) of the sounds. Compared with the JND of level in quiet, low level preceding sound did not significantly change the JND of the successive sound. However, moderate and high levels (ABL ≥40 dB) of preceding sound significantly increased the JND of the level of the successive sound. The JNDs were monotonically increased with increasing level of the preceding sound. When the preceding sound level was constant, the effect of preceding sound on the level discrimination of the successive sound was decreased with increasing level of the successive sound, and the effect was not significant when the level of the successive sound was high. These results were different from the previous findings in monaural conditions. Also, the present study did not found a significant effect of ILD difference between the preceding and the successive sound on the level discrimination of the successive sound.