We have developed the first prototype Micro-Insert device for microPET F-220. This device only has one detector module that consists of a LSO array of 12×12 crystals each measuring 0.86 (transverse) × 1.72 (axial) × 3.75 (radial) mm3 and a PS-PMT. The detector is mounted on a rotation stage and can be rotated to collect data from different angles. A bracket supporting the rotation stage is attached to the back of the scanner. Detector outputs are fed into one channel of the system electronics to establish the coincidence detection between the insert and the F-220. We also developed software to control the motion of the detector, communicate with the host PC of F-220 to acquire data, sort list-mode data, correct center of rotation error, and reconstruct images. We also developed 2D FBP and 2D ML-EM image reconstruction algorithms that model the special geometry of our insert system. The transverse and axial fields of view of the system are 3 cm and 2 cm, respectively. Image resolution of the insert system is 0.9-1.2 mm FWHM, a more than 40% improvement compared to the original system resolution of 1.8-2.1 mm for the F-220 system. A mouse injected with 18F- was imaged to show the significant improvement in image resolution using the insert system.