2D atomic crystals have been widely explored, usually owing to their numerous shapes, of which the typical hexagon has drawn the most interest. However, the relationship between shape and properties has not been fully probed, owing to the lack of a proper system. Here, we demonstrate for the first time the shape-dependent surface oxidation of 2D Mo2C crystals, where the elongated flakes are preferentially oxidized under ambient conditions when compared with regular ones, showing higher chemical activity. The gradual surface oxidation of elongated Mo2C crystals as a function of time is clearly observable. Structural determinations reveal that a discrepancy in the arrangement of Mo and C atoms between elongated and regular crystals accounts for the selective oxidation behavior. The identification of the shape-dependent surface oxidization of Mo2C crystals provides significant possibilities for tuning the properties of 2D materials via shape-control.