A simple method for the determination of trace amount of arsenic trihydride in gas phase was proposed. Arsenic trihydride was absorbed in sodium hypobromite and oxidized to arsenate. Molybdoarsenate-Malachite Green aggregate formed by Mo-MG reagent (mixed solution of ammonium molybdate and Malachite Green) was selectively collected on a nitrocellulose membrane filter (3μm pore size). The aggregate was dissolved in a small volume of methylcellosolve together with the membrane filter. The absorbance (λ: 627 nm) was proportional to the amount of arsenic trihydride. The molar absorptivity of aggregate in methylcellosolve is 2.6 × 105 M-1 cm-1. The method was applied to the determination of arsenic trihydride at 10 ng level in gas phase.