Experimental studies have shown that the gauche molecular conformations and the chain configurations play an important role in determining the structures and the phase transitions of the different rotator phases of n-alkanes. We extend our previous phenomenological theory to describe the effect of gauche defects and molecular flexibility on the various rotator phase transitions of n-alkanes. The roles of gauche defects and molecular flexibility are discussed by means of phenomenological theory. The effect of gauche defects and molecular flexibility is to modify the Landau coefficients and to shift the transition lines in the phase diagram. The theoretical predictions are found to be in good qualitative agreement with available experimental results.