A photocatalytic oxidation (PCO) reactor model was developed to analyze the removal of volatile organic compounds (VOCs) in indoor air. Two parameters, the fractional conversion, ε, and the number of mass transfer units (NTUm), were found to be the main parameters influencing the photooxidation performance of PCO reactors. The factors of affecting the effect of removing VOCs of PCO reactor are discussed. The predicted photocatalytic reaction rate coefficient, fractional conversion, ε, and NTUm for a typical PCO reactor agreed well with experimental data. The study shows that the effectiveness—NTU method used for heat exchangers is also an effective tool for designing PCO reactors and for evaluating their VOC removal effect.