β-NaYF₄:Eu³⁺microparticles co-doped with Gd³⁺ions were obtained by hydrothermal synthesis at 180 °C using citric acid as a stabilizing agent. All synthesized materials have a β-NaYF₄crystalline phase, where the unit cell volume increases upon the addition of Eu³⁺and Gd³⁺ions. The particles have a hexagonal prism shape and a size of 40-714 nm, where Eu³⁺and Gd³⁺doping results in size reduction. Upon 393 nm excitation, phosphors exhibit distinct emission peaks centered at 591, 615, and 695 nm and a weak band at 650 nm attributed to⁵D₀-⁷F_Jtransitions (J= 1-4). The optimum Eu³⁺doping concentration was found to be 30% in the NaYF₄host. Concentration quenching was realized through dipole-dipole interactions. Kinetic measurements showed a gradual decline of the⁵D₀lifetime from 6.7 ms to 2.2 ms along with an increase in Eu³⁺doping concentration. Co-doping of the small Gd³⁺number led to an increase of emission intensity and⁵D₀lifetime. The effects of Eu³⁺doping and Gd³⁺co-doping on radiative and nonradiative decay rates were studied using 4f-4f intensity theory.