In the field of biomedicine, upconversion nanoparticles have wide ranging applications from bioimaging to targeted cargo delivery, especially due to their excellent chemical and optical properties in comparison with conventional fluorophores. However, their use in biomedical applications is largely hindered due to strong absorption of short wavelength (<600 nm) light by biological tissues/cells and feeble luminescence. Hence, it is important to develop new strategies to increase the long wavelength (red) emission efficiency. In this work, we report an effective strategy to improve the red luminescence efficiency of NaYF4:Yb/Ho/Mn upconversion nanophosphors by varying the reaction conditions. The influence of different synthesis parameters, such as solvent ratio, reaction temperature, and reaction time, on the luminescence, crystal phase, and morphology of the upconversion nanophosphors has been studied in detail and optimized. The improvement in the crystallinity of nanophosphors is claimed as the main origin for the increase in the red emission intensity. This work could pave way for the versatile use of these bright red emitting upconversion nanophosphors in biomedical applications.