To study theoretically the relationship between the differential interference angle and the scattering angle in collisional quantum interference(CQI),we have investigated the differential interference angle of the atom-diatomic[case(a)]molecule system in detail.For the 2Π electronic state in Hund’s case(a),the degree of the differential interference is also discussed.The differential interference angles of NO(X2Π)are calculated quantitatively for the rotational energy transfer in Hund’s case(a)induced by collision with He,Ne and Ar atoms.The method to calculate the differential interference angle is presented.Several factors that affect the differential interference angle are investigated.Finally the variation of the differential interference angle with the impact parameter and relative velocity is discussed.