YOF: Yb³⁺, Er³⁺ powders with rhombohedral phase were successfully synthesized by using a modified sol–gel method with calcinating under a temperature range between 400 °C and 600 °C. A systematic study on the temperature-dependent morphology and spectroscopic properties of rhombohedral YOF: Yb³⁺, Er³⁺ powders have been performed. The YOF: Yb³⁺, Er³⁺ powders that prepared at 400 °C and 500 °C exhibit a plate growth habit, while the powder that prepared at 600 °C shows a sintering property. When excited by 980 nm laser beam, all of the rhombohedral YOF: Yb³⁺, Er³⁺ powders show efficient red light emission with light wavelength ranging from 630 nm to 700 nm. The upconversion mechanism and luminescence decay properties were discussed in detail.