Absorption of CW-$CO_2$ laser lines in $CH_3I$ molecule is investigated. Relative absorption coefficients are obtained by measuring optoacoustic signal from $CH_3I$ at the fill gas pressure of 6 torr.
Quantum number assignments for the absorption lines are carried out through numerical calculation, and the rotational quantum number of the upper and lower levels and vibrational modes of 12 absorption lines are identified. Eight lines are due to the transitions between v=0 (common ground state) and $v_6=1$ state, two lines are between v=0 and $v_3=2$ state, and the rest two lines are due to the transitions between $v_3=1$ and the exited combination state of $v_3=1$, $v_6=1$, respectively.
