The new crystal structure models of forms I and II of isotactic polybutene-1 have been proposed by analyzing the 2-dimensional X-ray diffraction data measured for the highly oriented samples of almost pure crystal forms. The crystal form I was found to take the hexagonal packing structure of the (3/1) helices with the space group P (3) over bar different from the previously reported R (3) over barc or R3c models. The right- and left-handed chains are packed alternately with the random directionality along the chain axis. The crystal form II was concluded to take the tetragonal unit cell of the (11/3) helical chains, the space group of which is P (4) over bar b2. The right-handed (left-handed) chains are positioned at one site with the statistical disorder of upward and downward directionality along the chain axis. The time-dependent electron diffraction measurement showed that the crystal lattices of forms I and II are related to each other with the common 110 plane boundary, as already reported by the other researchers. By referring to the crystal structures confirmed in this study, a new phase transition mechanism has been proposed for understanding this geometrical relation between these two crystal phases. The mechanism is based on a kind of soft mode concept; the mutually opposite translational movements of the right- and left-handed chains occur along the 110 plane of the tetragonal lattice of form II. The phase angle between the neighboring unit cells along the [110] direction is pi. This translational lattice vibrational mode increases the amplitude and causes the softening of the original form II unit cell into a transient structure composed of the hexagonally packed pairs of right- and left-handed chains. Then, this transient structure is stabilized to the crystal form I, during which the chain conformation changes cooperatively from (11/3) to (3/1) form by a slight change in the trans and gauche torsional angles of the skeletal chains. This newly proposed transition mechanism can explain also the formation of twin structure of form I crystals.