Phase transitions in micro-, submicro-, and nanocrystalline NaNbO3 were investigated by temperature-tuning Raman spectroscopy and X-ray powder diffraction method. Three powders with different average particle size showed successive phase transitions within the measured temperature range from -150 to 450 °C. The temperature characteristics of Raman active phonons in microcrystalline NaNbO3 corresponded the one reported for bulk NaNbO3, which transforms with increasing temperature from the ferroelectric N into the antiferroelectric P phase and finally above 373 °C (Tm3) into the antiferroelectric R phase. Submicrocrystalline NaNbO3, which takes the noncentrosymmetric orthorhombic Pmc21 structure at room temperature, transformed into a pseudocubic structure at 333 °C (Ts3). Nanocrystalline NaNbO3 showed a diffused phase transition from an orthorhombic Pmma structure to a high-temperature phase at around 180 °C (Tn2). For micro- and submicrocrystalline NaNbO3, hysteretic phase transition behavior was found for the temperature characteristics of specific phonons. On the other hand, the characteristics obtained for nanocrystalline NaNbO3 were much more diffused and did not show any hysteretic effect. Crystal structure refinements of the X-ray powder diffraction patterns using the Rietveld method demonstrated a hysteretic deformation of the a-b plane for microcrystalline NaNbO3 around Tm3 and of the b-c plane for submicrocrystalline NaNbO3 around Ts3. The temperature dependence of the primitive perovskite volumes showed a very small hysteresis for microcrystalline NaNbO3 but a clear one for submicrocrystalline NaNbO3. Lattice distortion of the submicrocrystalline Pmc21 structure from a cubic perovskite lattice induced a particularly large contraction of parameter c around Ts3 with increasing temperature, which resulted in a decrease of the primitive cell volume. This transition showed a first-order type character, which may relate to a ferroelectric-antiferroelectric transition. Rearrangement of the NbO6 octahedra induces a transition from an orthorhombic into a pseudocubic structure.