Luminescent properties of Er³⁺- and Yb³⁺- co-doped CaF₂ upconversion nanoparticles (UCNPs) were investigated in single particle and densely-packed states with a custom-built microscope. The single UCNPs exhibit linear dependency of luminescent intensity on excitation power while the densely-packed UCNPs exhibit a 2-order power law-dependency indicating a two-photon absorption process. Time-domain luminescence intensity measurements were performed and the curves were fitted to excitationnemission rate functions based on a simplified three-state model. The results indicate that the intermediates in single particles are much less and saturated in a short time, and there are strong couplings of the ground states and intermediate states between neighboring UCNPs in densely packed UCNPs.