A miniaturized chemical vapor sensor probe was developed using a porous glass microsphere (PGM) as the alignment-free optical microresonator. The porous microsphere was placed inside a thin wall silica capillary tube that was fusion-spliced to an optical fiber. The whispering gallery modes (WGMs) of the microsphere were excited by the evanescent field of the light propagating inside the capillary thin wall. Adsorption of chemical vapor molecules into the pores led to a refractive index change of the PGM and thus the resonance wavelength shift of the WGMs. To facilitate the in-taking of chemical vapor molecules into the PGM, a micro window was opened at the backend of the capillary tube using femtosecond laser micromachining. Ethanol vapor was used to demonstrate the probe for chemical vapor sensing. With a miniaturized size, integrated structure and reflection mode of operation, the proposed probe may find useful in many practical applications such as environmental monitoring and biomedical sensing.