A transmission monitoring method and apparatus are provided. The method includes: in a case that an input port of a network node receives M frames, counting the number N of frames sent by an output port of the network node corresponding to the received M frames; in a case that the M is different from the N, determining that frame loss exists in the network node, wherein the M and the N are positive integers. Through the method and apparatus of the present document, the problems of failing to determine a specific device where the frame loss occurs and the construction cost being high are solved, the accuracy and the flexibility of monitoring a position of the frame loss are improved, and the construction cost is reduced.