We report a comprehensive study of the structural, morphological, and optical properties, and UC-based ratiometric temperature sensing behavior of (α) cubic and (β) hexagonal phases of NaYF₄:Yb³⁺/Er³⁺ nanoparticles. The α-NaYF₄:Yb³⁺/Er³⁺ and β-NaYF₄:Yb³⁺/Er³⁺ nanoparticles were synthesized using co-precipitation and hydrothermal methods, respectively. Powder X-ray diffraction studies confirmed the phase purity of the samples. The morphological studies show uniform particle sizes of both phases; the average particle size of α-NaYF₄:Yb³⁺/Er³⁺ and β-NaYF₄:Yb³⁺/Er³⁺ was 9.2 nm and 29 nm, respectively. The Raman spectra reveal five sharp peaks at 253 cm⁻¹, 307 cm⁻¹, 359 cm⁻¹, 485 cm⁻¹, and 628 cm⁻¹ for β-NaYF₄:Yb³⁺/Er³⁺, whereas α-NaYF₄:Yb³⁺/Er³⁺ shows two broad peaks centred at 272 cm⁻¹ and 721 cm⁻¹. The optical property measurements show that α- and β-NaYF₄:Yb³⁺/Er³⁺ phases have distinct upconversion emission and temperature sensing behavior. The upconversion emission measurements show that β-NaYF₄:Yb³⁺/Er³⁺ has higher overall emission intensities and green/red emission intensity ratio. The temperature-dependent upconversion emission measurements show that α-NaYF₄:Yb³⁺/Er³⁺ has higher energy separation between ²H_11/2 and ⁴S_3/2 energy states. The temperature sensing performed utilizing these thermally coupled energy levels shows a maximum sensitivity of 0.0069 K⁻¹ at 543 K and 0.016 K⁻¹ at 422 K for β-NaYF₄:Yb³⁺/Er³⁺ and α-NaYF₄:Yb³⁺/Er³⁺, respectively.