The melting point maximum against pressure is reported for poly(4-methyl-pentene-1) : 270 degrees C at 1.5 kbar. X-ray diffraction at high pressures shows no transformation up to 15 kbar at room temperature; the long-range order in the crystal remains unchanged with pressure while disorder of the first kind increases to cause the increase of the internal energy of the crystal. The origin of the melting point maximum is discussed on the basis of the low packing coefficient of helical polymers with bulky side groups and the long-range order maintained up to the melting point at high pressures.