Abstract: Collision-induced rotational energy transfer in the A~2Σ~+(v=0) state of NO has been measured by two-color double resonance-multiphoton ionization spectroscopy. Rotationally resolved relaxation spectra have been obtained for initial rotational levels N_i=1,7,13. The state-to-state rotational energy transfer rate constants have been determined from the experimental spectra by means of the rate equations under single collision approximation. The rate constant decreases with increasing rotational-quantum-number change ΔN. Exponential- and power-gap scaling laws have been used to analyze the experimental results and it has been found that the power-law fitting is better than the exponential-law fitting, especially for N_i=1. It has to be pointed out that the relaxation spectra from different initial ortational levels cannot be fitted well with a single set of parameters for either energy-based law. This results indicates that in addition to the energy gap some factors related to molecular rotation, e.g. the rotation period to the collision time ratio, must be taken into consideration in the collision model in order to describe the rotational energy transfer in detail. Similar rotational relaxation spectrum has been observed in NO-Xe (1:4) collision system. R-T energy transfer is considered to be the dominant pathway in collision-induced energy transfer between NO* and NO molecules or between NO* molecule and Xe atom. In addition, the rotation relaxation in the B∏(O_u~+) state of I_2 has been studied by OODR-MPI and the observed spectral line broadening discussed.