The St Pierre and Miquelon Archipelago hosts the only French Leach's Storm-Petrel (Oceanodroma leucorhoa) colony. We conducted a survey during the 2008 breeding season to estimate the breeding population size on Grand Colombier Island. This survey included an estimation of burrow detection probability using a double-observer approach. We estimated that 3% of Leach's Storm-Petrels nests had failed before we started the survey. Nest occupancy probability was neither affected by slope nor vegetation type and was 0.546 ± 0.029. Burrow density was positively affected by slope and, consequently, was much lower on the plateau than on island slopes. Burrow detection probability was neither affected by observer nor by habitat and was 0.89 ± 0.01. We estimated the population to be 363,787 [95% CI  =  295,502–432,072] breeding pairs, which is among the largest Leach's Storm-Petrel colonies in the northwestern Atlantic Ocean.