Abstract Carbon nanowalls (CNWs) provide a high-surface-area platform for enzyme immobilization and are promising for bioelectrochemical devices. This study investigates the effect of CNW height on enzyme utilization and performance in CNW-based glucose biofuel cells. CNWs with controlled heights were synthesized by inductively coupled plasma chemical vapor deposition, and glucose oxidase (GOD) was immobilized via surface functionalization. The amount of immobilized GOD increased approximately linearly with CNW height. Electrochemical measurements confirmed that the immobilized enzymes retained catalytic activity and functioned as efficient bioanodes. However, although the maximum power density increased with CNW height, the increase became progressively smaller despite continuous enzyme loading, indicating that increasing enzyme loading does not necessarily lead to a proportional increase in power generation. A schematic model suggests that enzymes in deeper regions are not effectively utilized because of mass transport limitations. These results provide design guidelines for optimizing three-dimensional nanocarbon electrodes for enzymatic bioelectrochemical devices.