The Parsons-Lee theory has been modified to study the liquid-crystalline phase behaviour of the linear fused hard sphere chain fluids. The modification of the Parsons-Lee theory is based on the application of the so-called effective molecular volume instead of the real molecular volume. This alteration does not mean any change for the Parsons-Lee treatment of the hard convex bodies but does for the non-convex ones. The results of the modified Parsons-Lee theory show very good agreement with simulations not only for the location of the isotropic-nematic phase transition but for the equation of state.