By using a special double-layer membrane to avoid interaction among flowing fields in different pores．the theoretically predicted discontinues transition in ultra-filtration of flexible linear polymer chains was observed for the first time．Namely

the chains could pass thmugh pores with sizes much smaller than the chain’s unperturbed radius when the flow rate is higher than a threshold．In theory

such a threshold is independent of both the chain length and the pore size．However．we found that when the macroscopic flow rate Was lower than the threshold．some of the pores were blocked by a fraction of longer polymer chains．Therefore

the microscopic flow rate in the non-blocked pores became higher than the threshold so that the rest of the chains could pass throllgh these non-blocked pores．This leads to an apparent separation of the chains with different lengths.