Traffic flow measurement is essential to implement QoS control in the Internet. Flow monitoring system collects and stores sampled flow states in a flow table (FT) and the entries are renewed at every packet sampling. Entries in the FT are checked and removed when no packets are sampled within a predetermined timeout. We propose an efficient timeout checking mechanism based on checking a small number of entries selected randomly from the FT. Our proposed method aims to reduce the number of memory accesses dramatically and keep the memory size small. We evaluate our method and compare with the conventional method that checks all flow entries of the FT periodically. Our simulation and comparison results show that our method is able to reduce the number of memory access at a factor of 1000 with a small increase in memory size of approximately 10 percent.