We report, for the first time, the effort to one-step micromachine an optical fiber inline square resonator by femtosecond (fs) laser ablations. Such an inline square resonator is intended for infield chemical and biological sensing applications. Our preliminary data shows the feasibility of directly machining three dimensional microstructures on a silica optical fiber with a diameter of 125μm. The observed low quality (Q) factor can be further improved by post annealing and chemical etching processes that smooth the surface roughness.