Abstract The microwave spectrum of cyclohexanone has been investigated in the frequency region from 7.7 to 23.4 Gc/sec. The R- and Q-branches for both the a- and the c-type have been assigned for the vibrationally ground state. The rotational constants obtained are A=4195.30 Mc/ sec, B=2502.57 Mc/sec, and C=1754.49 Mc/sec. From these values it can be concluded that the molecules have the chair form. The value of the component of the dipole moment |μa| is 2.74±0.03 D, and that of |μc| is 0.86±0.03 D. The value of the total dipole moment is 2.87±0.04 D, a value which is in agreement with that obtained by Debye’s method. For the structural parameters Romers’ data obtained by the electron diffraction seems to be nearly correct.