In this work, Tantalum pentoxide (Ta2O5) microparticles were achieved by one-step thermal oxidation of Tantalum (Ta) foils under 600 °C for 6 h. The effect of time and temperature was explored in the process (50 to 600 °C, 1 to 6 h). Thus, Ta foil was chemically cleaned with nitric and hydrochloric acid and then was immersed in water; finally it was dried with Nitrogen flux. The foils were isothermally oxidized in a horizontal furnace with 21 wt. % Oxygen atmosphere in a constant flux of 20 L/min. The x-ray diffraction, scanning electron microscopy, Raman and energy dispersive x-ray spectroscopy led to the conclusion that formation mechanism of microparticles is mainly constituted by lattice parameters mismatching between Ta and Ta2O5 and crystallite size difference. Along the oxidation process at 600 °C, the phase transformation across time carries defect concentrations on entire material experimentally demonstrated by Ta2O5 Raman shift and then the sample is broken in irregular shape. As oxidation time reaches 6 h value, phase transformation in Ta foil causes mismatching that firstly diminishes crystallite size, then evolves to cracks and finish by the formation of the particles in the micrometer order size.