We model and demonstrate resonantly enhanced third harmonic generation in microfiber loop resonators, in which the large pump field intensity is exploited to improve the conversion on resonance. Silica microfibers were fabricated with waist diameters near 0.76 μm to ensure intermodal phase matching. When pumped with λ=1.55 μm 4 ns pulses at 100 W peak power, the conversion efficiency is 3×10(-6) over an estimated interaction length of ∼1 mm near the waist. The resonator is then formed by manually translating and twisting the microfiber ends to produce loop diameters down to 6 mm and resonant enhancements up to 7.7 dB for the same pump parameters.