Formyl radicals, DCO, were produced in solid CO matrix and studied by EPR using gas deposition technique on a cold substrate at 4.2 K. The radical electron spin-lattice relaxation was measured to be anisotropic. The anisotropy was found to originate from the anisotropic orientational motion of the radical with the most intensive librations performed around a z axis corresponding to the smallest moment of inertia of the molecule. It was shown that, in solid CO, the libration motion of small molecules is modeled by considering a particle rotating in the orientational potential box. A suggestion was substantiated about the complicated nature of the orientational motion of HCO in the solid CO matrix including the intensive librations and slow tunneling rotation of the molecule around the z axis. Keywords: electron paramagnetic resonance, matrix isolation, formyl radical, anisotropy of the spin-lattice relaxation time, librations and tunneling rotation of molecules.