The reactivity of chromium oxide cluster anions (CrO<sub>3</sub>)<sub>1−4</sub>O<sup>−</sup> towards low carbon (C<sub>1</sub>−C<sub>4</sub>) alkanes has been experimentally investigated at 298 K by employing a homemade ship-lock-type reactor coupled with a time-of-flight mass spectrometer. The results demonstrate that while CrO<sub>4</sub><sup>−</sup> and Cr<sub>2</sub>O<sub>7</sub><sup>−</sup> clusters could abstract a hydrogen atom from C<sub>2</sub>H<sub>6</sub> and CH<sub>4</sub>, respectively, Cr<sub>3</sub>O<sub>10</sub><sup>−</sup> and Cr<sub>4</sub>O<sub>13</sub><sup>−</sup> clusters were found to be inert towards <i>n</i>-C<sub>4</sub>H<sub>10</sub> within the detection limit. Theoretical results reveal that CrO<sub>4</sub><sup>−</sup> and Cr<sub>2</sub>O<sub>7</sub><sup>−</sup> clusters possess delocalized chromium-bonded oxygen radicals (Cr−O<sup>−•</sup>), which rationalizes the hydrogen atom abstraction reactions between (CrO<sub>3</sub>)<sub>1,2</sub>O<sup>−</sup> clusters and alkanes. However, the active sites of (CrO<sub>3</sub>)<sub>3,4</sub>O<sup>−</sup> clusters evolve to peroxide species (O<sub>2</sub><sup>2−</sup>), which exhibit inferior activity compared to O<sup>−•</sup> radicals. The increase of Cr-3d orbital energy driven by the more negative charge around CrO<i><sub>y</sub></i> unit formed via downsizing the cluster size has been proposed to account for favorable reduction of O<sub>2</sub><sup>2−</sup> and selective generation of reactive O<sup>−•</sup> radicals in small-sized (CrO<sub>3</sub>)<sub>1,2</sub>O<sup>−</sup> clusters. This study not only enriches the chemistry of metal-bonded O<sup>−•</sup> radicals toward alkane activation under mild conditions, but also provides a new insight into the conversion between O<sub>2</sub><sup>2−</sup> and O<sup>−•</sup> radicals over metal oxides.