The choice of a laser gain medium is crucial in achieving efficient and high-power outputs of optically stimulated WGM microcavity lasers. This work employs an Yb:YAG crystalline film as the gain medium for the microdisk laser. The Yb:YAG crystalline film is exfoliated from a bulk of a Yb:YAG crystal by the ion-implantation-enhanced etching method. The crystalline film is shaped into a microdisk through focused ion beam milling. This Yb:YAG microdisk laser achieves a single-mode laser output (with a side-mode-suppression ratio of 27.8 dB) under a 946 nm laser pumping. The maximum slope efficiency reaches 27% with a maximum output power of 1.1 mW.