The microwave spectrum of mono deuterated methyl formate (HCOOCH2D) was measured in three regions by Fourier transform microwave spectroscopy, conventional Stark modulation techniques, and submillimeter wave spectroscopy. Effects due to the large amplitude torsional motion of the partially deuterated CH2D methyl group were evidenced and lead to two sets of transitions. Both sets display a rigid-rotator behavior. In one of them, a small tunneling splitting ranging from 0 to 20MHz was observed. This splitting was accounted for using a phenomenological IAM-like treatment accounting for its rotational dependence. This model was used to analyze line frequencies of the 1388 measured microwave lines. The unitless standard deviation of the fit is 1.5 and the spectroscopic parameters obtained are consistent with the structure of the molecule and the nature of the large amplitude tunneling motion.