The problem of excess capacity has persisted in many fisheries worldwide, threatening the sustainability of fisheries, even in cases where there are regulatory restrictions on fishing inputs. Getting fishers involved in monitoring illegal fishing has been one of the solutions proposed to tackle excessive investment in fishing capacity. However, the real effects of participation in monitoring have not yet been investigated in a rigorous and quantitative way. Using survey data for small-scale fisheries under a Territorial Use Rights for Fisheries (TURFs) system in Vietnam, this study measures the extent of excess capacity at individual fisher level using a bootstrapped Data Envelopment Analysis and investigates the effects of fisher participation in monitoring on level of excess capacity by employing an Endogenous Switching Regression model. We find evidence of substantial excess capacity even under TURFs, with fishers on average operating at 59% capacity. However, the results also show that participation in monitoring contributes to a significant reduction in excess capacity. Non-monitors are likely to have short-sighted investment behavior while monitors are likely to be driven by perception about the management of TURFs, and long-term incentives in their investment behavior. Age, education, and alternative livelihood also help explain the variation in the level of excess capacity among fishers. Knowledge of the drivers of fisher investment are of critical importance to policy-makers to establish effective capacity management interventions. Engaging fishers in monitoring should be considered as one of the key strategies for reducing excess capacity. To promote fisher participation in monitoring, effort should be directed towards enhancing awareness of the consequences of illegal fishing and building fisher trust in peers and local government officials.