B4C boron carbide thin films deposited by radio frequency magnetron sputtering in the temperature ranging from room temperature to 650 A degrees C are amorphous. In this article, pair distribution function (PDF) and Fourier transform infrared (FTIR) spectroscopy were used to characterize the short range order (SRO) structure of amorphous B4C thin films. FTIR spectra indicated that icosahedrons exist in the amorphous B4C thin films. The existence of icosahedrons was further verified by the PDFs of amorphous B4C thin films, which were derived from digital selected area electron diffraction patterns. Furthermore, by comparing the PDFs of amorphous B4C thin films with those of three crystalline boron modifications and three structural models of boron carbides (B4C or B10C2), the SRO structure of amorphous B4C thin film was revealed to be similar to that of beta-rhombohedral boron, but with the peak shifted to shorter distance.