We investigated the effects of tension induced by micropipet aspiration on giant unilamellar vesicles (GUVs) composed of dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC). We analyzed the time course of the fraction of intact GUVs among all of the GUVs under constant tension sigma and obtained the rate constants of pore formation k(p)(sigma). To determine k(p) we developed an approach using the mean first passage time. The fitting of the theoretical curves of k(p) versus sigma to the experimental data determined the line tension of a prepore, Gamma. The value of Gamma of a DOPG/DOPC bilayer was smaller than that of a DOPC bilayer.