The "irreversibility line" of high Tc fluorine-doping Bi(Pb)SrCaCuO(F) bulk superconductor has been studied experimentally. It is shown that there are different H*∝(l-t)3/2 relations in various regions of magnetic fields, namely, H*=1590(1-t)3/2 in the field region below 120 Oe and H*= 35700(1-t)3/2 -2480 in the region above 120 Oe. We think that the irreversibility lines in low field and in high field represent the behaviour of weak link and the grain superconductivity respectively. We note that H*(T) is substantially a melting line of the flux lattice. It is in the vortex lattice state below the "irreversibility line" H*(T) and in the vortex liquid state between H*(T) and H(c2) (T) curves.