It has been demonstrated that biofilm reactors are effective in removal of phenol from wastewaters. As a new reactor type, hollow fiber biofilm reactors (HFBRs) have been applied to wastewater treatment and models predicting the activity of the biofilms in HBBRs would be beneficial in understanding the behavior of the reactor under varied conditions. In this study, we modeled phenol treatment in a hollow fiber biofilm reactor. We applied the maximum substrate consumption approach, which was previously developed by our research group, to predict biofilm density and to model phenol treatment in the hollow fiber biofilm reactor. The model assumes biofilms as a goal seeking system and hypothesis that biofilms arrange their structure to optimize phenol consumption rate within the active biofilm. The model solution predicted the biofilm density, active thickness, effectiveness factor, and phenol consumption rate using lumen phenol concentration as the only independent parameter. In addition, we evaluated the effect of radial flow velocity on the predicted parameters. The biofilm density exponentially decreased with increased lumen phenol concentration, while the active biofilm thickness exponentially increased. The maximum biofilm density was predicted as 110 g/L showing good correlation with literature data. The phenol consumption rate increased with the lumen concentration without being affected by phenol inhibition. However, the phenol consumption rate decreased sigmoidally with increased biofilm density. The results of this study showed that the performance of the proposed HFBR reactor depended on the lumen phenol concentration and radial flow velocity in the biofilm.