In natural environment, human and animals often percept signal sound stimuli in background noise conditions. However, it is not clear how the low-level weak background noise affect the coding of frequency tuning of auditory cortical neurons. In the present study, we determined how weak background noise affected the frequency response area of 79 neurons in the rat primary auditory cortex. The results demonstrated that weak background noise either suppressed or facilitated the responses of some cortical neurons to sound stimuli. Generally, the suppression of responses by weak background noise reduced the extent of frequency tuning and the extent of best frequency response area of auditory cortical neurons, and the facilitation of responses by weak background noise enlarged the extent of frequency tuning and the best response areas of the affected neurons. For a minority of neurons, weak background noise did not significantly change their extent of frequency tuning, but changed their extent of best frequency response area. Weak background noise did not change the characteristic frequency of 63.64% of the sample neurons and the minimum threshold of 55.84% of the sampled neurons. The tip of the frequency tuning curves of auditory cortical neurons was more susceptible to be affected by the background noise than the middle part of the frequency tuning curves of these neurons.