We experimentally demonstrate an optical microfiber loop resonator. The resonator was formed in free space by creating a loop from the subwavelength-diameter waist of a short biconical optical fiber taper. The loop length was chosen so that the free spectrum range of the resonator was ∼100 GHz at the optical communication wavelengths near 1.5 μm. In order to change and optimize the spectral characteristics such as the effect of birefringence, the shape of resonances, and the free spectrum range, we manually varied the microfiber self-coupling by alignment of the input and output ends of the loop, which were attached to each other by Van der Waals and electrostatic forces. In particular, we tuned the microfiber loop resonator to exhibit resonances with a Q-factor exceeding 15 000 (finesse ≈10) and, also, to the regime of critical coupling with the extinction ratio of transmission oscillations exceeding 34 dB. This paper was in press when we achieved the values of 95 000 and 630 000 for the loaded and intrinsic Q-factor, respectively (see note added in proof). We believe that the demonstrated Q-factor can be significantly enhanced with the more uniform microfiber.