To interpret theoretically our previous experiment [Chem. Phys. Lett. 365 (2002) 244], a theoretical model is presented, based on the time-dependent first order Born approximation, taking into account the anisotropic Lennard-Jones interaction potentials and 'straight-line' trajectory approximation. Quantum interference of Pi-state diatomic molecules, which originates from the difference between the two A-related collision potential energy surfaces, is studied theoretically. Using our derived results, three phenomena are interpreted: (1) cross-sections sigma(Pi(+)) > sigma(Pi(-)), (2) the propensity of +/- parity-conservation, and (3) the abnormal phenomenon of sigma(DeltaJ=0)(epsilon-->epsilon') < sigma(DeltaJ=+/-1)(epsilon-->epsilon') for He and Ne. (C) 2003 Elsevier Science B.V. All rights reserved.