We have studied the in influence of the mean molecular length on the nature of the cholesteric (N*) - smectic A (A) liquid crystal phase transition for thermotropic compounds of cholesteryl myristate (C14), cholesteryl nonanoate (C9), and binary mixtures of C14 - C9 and cholesteryl caproate (C6) phase (dA), the mean molecular length (<img src="http:/img/fbpe/bjp/v32n2b/07eq01.gif">) in the N* domain, and the correlation length (epsilon), as well as changes along the N* - A phase transition, were determined from the X-ray diffraction data. The results show a decrease in A phase domain and an expansion of the N* - domain as the mean molecular length is diminished. Such effect induces a second order A - N* phase transition for a particular mixture of 63.1 mol% of C9 and 36.9 mol% of C6, evidenced by viscosity measurements in good agreement with previous density data.