The microwave spectrum of cyclopropylphosphine-borane, C3H5PH2-BH3, has been investigated in the frequency range 150-195 GHz. The spectral assignment was supported by high level ab initio calculations. Two stable conformations have been predicted: the most stable antiperiplanar form and synclinal form that is higher in energy by 7.3 kJ/mol. In the observed spectra, only the most stable antiperiplanar (ap) form has been assigned. The analysis of the rotational spectra in the lowest excited vibrational states of the ap conformer has enabled determination of the potential function for the C P torsional mode in the vicinity of equilibrium position. The barrier to internal rotation of the BH3 top has been determined to be 9.616(15) kJ/mol and agrees well with quantum chemical calculations.