β-NaYF4:Yb3+,Er3+ microcrystals of good shape and with various aspect ratios were successfully prepared using a facile hydrothermal method. Field emission scanning electron microscope images and X-ray diffraction patterns were used to characterize the resulting samples. The upconversion luminescence (UCL) properties of the products were investigated as a function of their aspect ratios. It was found that the proportion of green to red emission intensities increased from 0.8 to 1.4 in the range of aspect ratios 0.04 to 0.9. This study demonstrates a new strategy for the synthesis of UCL materials with different spectral properties simply by changing their aspect ratios.