The ratio between the reactivity of HC and NOx, i.e., \(\Phi = \sum {k_{OH + HC_i } [HC_i ]/(k_{OH + NO} [NO] + k_{OH + NO_2 } [NO_2 ])} \), is used to study the chemistry of HOx radicals (HO2 and OH) in the marine boundary layer (MBL). Data analysis shows that HOx production and OH/HO2 ratio strongly depend on Φ. The OH radical concentration presents an ascending tendency in low Φ region but a declining one in high Φ region, while HO2 radical increases with increasing Φ under low Φ conditions but appears to be nearly constant with the variation of Φ under high Φ conditions. The observed OH/HO2 ratio shows a continuous decrease with the increase of Φ. Model studies are conducted to illuminate the behavior of HOx radicals in different Φ regions, and the results represent general agreement with the observed variations of HOx radicals and can well explain the observed behavior of HOx radicals. This study demonstrates that Φ could be used as an efficient parameter to ascertain the behavior of HOx radicals in the clean marine boundary layer.