FT-IR Spectroscopy have been used for identifying both the structure of BN and the intensity of the compressive stress in cubic boron nitride (c-BN) film prepared by a unbalanced of (13.56 MHz) magnetron sputtering of a hexagonal boron nitride target in a mixture argon and nitride discharge. A T(temperature) - V(negative bias) phase diagram was obtained using the phase structure identify by IR spectra. Comparing the reflection infrared spectra (RIR) with the transmission infrared spectra (TIR) measured from same c-BN film, it is firstly found that RIR peak position of c-BN is lower than TIR peak position of c-BN, this means that the compressive stress on the surface layer of c-BN film is smaller than that inside c-BN film, may be this is the reason why thicker c-BN film can not be synthesized. A higher IR peak position of 1064 cm−1and a lower peak position 1004.7 cm−1were detected from a broken and partly peeled off c-BN film. The peak position of 1064 cm−1agrees with that of bulk c-BN at 1065 cm−1which was synthesized at high temperature and high pressure, while the peak position of 1004.7 cm−1accords well with the result calculated (1004 cm−1) by Wentzcovitch and it may be closes to that of the stress free value of c-BN. Using the result measured by Ulrich, the shift rate of IR peak position of c-BN in the films is about 3.8 cm−1/Gpa to be obtained.