We demonstrate experimentally an air-slot mode-gap photonic crystalcavity with quality factor of 104 and modal volume of 0.02 cubic wavelengths, based on the design of an air-slot in a width-modulated line-defect in a photonic crystal slab. The origin of the high Q air-slot cavity mode is the mode-gap effect from the slotted PhCWG mode with negative dispersion. The high Qcavities with ultrasmall mode volume are important for applications such as cavity quantum electrodynamics, nonlinear optics, and optical sensing.