457 nm and 915 nm continuous lasers were used to study the Yb3+ near-infrared luminescence and up-conversion energy transfer mechanisms in YAG:Ce, Yb phosphors synthesized via solid-state reaction and sol-gel method. Under either excitation, sol-gel samples demonstrated the same Yb3+ near-infrared luminescence quenching concentration of -5%, with similar trends. In solid-state reaction samples, 915 nm excitation yielded a quenching concentration of -5%, while 457 nm excitation resulted in a quenching concentration of 10%, indicating a less homogeneous distribution of dopant ions. Visible up-conversion luminescence from Yb3+-impurity ion and Yb3+-Yb3+ ion pairs can be observed. Solid-state reaction samples demonstrate stronger cooperative luminescence from Yb3+-Yb3+ ion pairs. The results show that clustering of Yb3+ ions is more significant in samples synthesized via solid-state reaction, which could lower the near-infrared down-conversion efficiency of this type of material.