In situ and real-time observation of 2-dimensional grazing incidence x-ray diffraction (2D-GIXD) during growth of pentacene thin films were carried out using a newly home-built portable vacuum deposition chamber using synchrotron radiation at SPring-8. Crystal growth and successive polymorphic transformation from thin film phase to bulk phase are clearly observed at room temperature and 75 degrees C. A distinct orientation of bulk phase characterized by tilted (001) plane is found in the grown thin films at room temperature.