People suffering from developmental dyscalculia (DD) show an abnormal pattern of the size congruity effect. They do not display a facilitation component in a numerical Stroop task. In this task, participants are presented with 2 digits that differ both in physical size and numerical value, and they have to compare the digits while ignoring one of the dimensions. The present study examined performance of those with DD and control participants in the numerical Stroop task under cognitive load. The no-load condition replicated previous findings (i.e., lack of facilitation in the physical task for the DD group). Load had opposite effects on interference and facilitation. Load eliminated facilitation and increased interference in the control group. Load increased interference only in the physical task in the DD group. The opposite effect of load on facilitation and interference suggests that these components are related to different cognitive mechanisms. The fact that load produced a DD-like pattern in the control group could suggest that individuals with DD suffer from difficulty in recruiting attention in addition to the deficits in numerical processing.