Amplified spontaneous emission (ASE) characteristics of a green fluorescent dye (10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1] benzo- pyrano [6,7,8-ij]quinolizin-11-one) (C545T) encapsulated in a highly ordered three-dimensional (3D) inverted-opal titania ( $\mathrm{Ti}{\mathrm{O}}_2$ ) photonic crystal (PC) microcavity were studied. Due to the utilization of a $\mathrm{Ti}{\mathrm{O}}_2$ PC, the emission spectrum was greatly narrowed and the ASE threshold, gain, and loss were significantly improved. The threshold, gain, and loss reached $1.25\mathrm{mJ}{\mathrm{pulse}}^{-1}{\mathrm{cm}}^{-2}$ , ${34.69\mathrm{cm}}^{-1}$ , and ${16.9\mathrm{cm}}^{-1}$ , respectively. The possible reason for the improvement in the ASE performance by the PC is attributed to the 3D photon localization by the microcavity effect of the PC.