Block copolyetheresters with hard segments of poly(pentamethylene p,p’-bibenzoate) and soft segments of poly(tetramethylene ether) have been prepared by melt polycondensation of dimethyl p,p’-bibenzoate, 1,5-pentanediol and poly( tetramethylene ether) glycol(PTMEG) with molecular weights of 650, 1000 and 2000. The polymer composition is governed by the charge molar ratio(x) of PTMEG to dimethyl p,p’-bibenzoate. The block copolyetheresters with x=0.05, 0.1 and 0.2 display a monotropic smectic phase due to the poly(pentamethylene p,p’-bibenzoate) segments. But the block copolyetheresters with x=0.3 exhibit no liquid crystalline behavior. The molecular weight of the PTMEG used has significant effect on the glass transition temperature and crystallizability of the polyether segments. It can be seen from the glass transition temperature results that the miscibility between amorphous parts of the polyether segments and those of the polyester segments is also dependent on the molecular weight of the PTMEG used.