We observed the crystal structure changes of rubrene (5,6,11,12-tetraphenylnaphthacene) polycrystal thin films on SiO(2)/Si(100) substrates at various heat-treatment temperatures by X-ray diffraction, and a near-field microwave microprobe technique. An amorphous rubrene thin film was initially observed at heat-treatment temperature of 35 degrees C. After the treatment with in-situ vacuum post-annealing at 80 degrees C for 22 h, the rubrene thin film was transformed from the amorphous phase into a crystalline phase of orthorhombic structure. We could obtain a higher field effect mobility of 0.047 cm(2)/V.s and lower threshold voltage of -4 V for the following heat-treatment process: pre-annealing at 80 degrees C, cooling at 40 degrees C, and post-annealing at 80 degrees C for 22 h. (C) 2011 Elsevier B.V. All rights reserved.