Abstract: Using LIF technique, the distribution of rotational temperature of I_2 along the supersonic jet axis has been measured. I_2 was seeded in He, Ar and N_2. Results show that rotational relaxation appears very efficient in jets and Ar and N_2 as car- rier gases are more effective in rotational relaxation than He. The effect of “ve- locity slip” is calculated for I_2 seeded in He and Ar. With He as carrier gas “ve- locity slip” is as large as 360 ms~(-1) compared with 20 ms~(-1) in Ar case, It is pointed out that “velocity slip” would be responsible for the inefficiency He in rotational relaxation of I_2 in our case.