The rotational spectrum of the cyanomethyl radical CH2CN in its B-2(1) ground electronic state was studied by microwave spectroscopy using a source-modulated spectrometer equipped with a free-space cell. The CH2CN radical was generated in the cell by dc-glow discharge in pure CH3CN, and CD2CN in CD2CN. Fifty a-type R-branch rotational transitions were measured in the frequency region of 80 to 282 GHz, where fine structure and hyperfine structures due to the nitrogen and hydrogen nuclei were resolved for lower-N transitions, but only the fine structure for higher-N transitions. Similarly fine-structure resolved rotational transitions were measured for CD2CN in the frequency region of 190 to 286 GHz. The molecular constants of CH2CN, including the fine-structure constants and hyperfine coupling constants due to both the nuclei of N and H, were precisely determined by least-squares methods from 146 observed spectral lines, including 36 astronomically observed lines for the K-a=0 components of N=1-0 and 2-1 from a dark cloud TMC-1 [Kaifu et al., Astrophys. J. Suppl. (submitted 1997)]. Those of CD2CN were similarly observed from 92 observed spectral lines. The rotational constants of both the species were used to derive molecular structural parameters of CH2CN : r(0)(CN)=1.1919(13) Angstrom, r(0)(CC)=1.3680(12) Angstrom, and r(0)(CH)=1.0894(7) Angstrom on the assumption of 120.22 degrees for angle HCH. The spin densities of unpaired electrons estimated from the hyperfine coupling constants for the N and H atoms are consistent with the molecular structure obtained.