We carried out detailed structural characterizations across the structural transition in SrRuO3 epitaxial thin films grown on SrTiO3 (001)(pc) substrate. The fabricated films undergo a structural transition from the low-temperature orthorhombic phase to the high-temperature pseudocubic phase at 280 degrees C. We find that, for films thinner than 20 nm, the transition becomes broader while thicker films display a sharp transition. Detailed X-ray diffraction measurements including reciprocal space mappings at various temperatures also reveal that the thinner films have a distorted orthorhombic unit cell resulting from the strain-induced additional rotation in the RuO6 octahedra, while, for the high-temperature pseudocubic phase, the film structure remains the same irrespective of film thickness. The results strongly suggest that the substrate-induced strain has a strong influence on the RuO6 rotation pattern in the epitaxial thin films