The effects of queuing delay of output buffer on the property of crosstalk in optical packet switching nodes are investigated. The analytical relationship between crosstalk and buffer length is derived. It is concluded that the crosstalk increases with the increase of buffer length. Among the crosstalk sources, the most of them are incoherent, and the probability of coherent crosstalk decreases with the increase of buffer length significantly, and if the buffer length is longer enough, the coherent crosstalk could be neglectable.