Courtyard vernacular underground dwellings most commonly called "dig pit houses" , have been for centuries a shelter for the human kind. Despite their noticeable energy saving properties and security enhancing design features, they present several disadvantages related to indoor environment quality such as poor indoor natural daylighting. The dig pit in these typologies is the main design feature that allows natural light into the rooms. In this paper the effect of changing the courtyard’s geometry on the daylighting performance inside the rooms is investigated by the mean of computer simulations using Ecotect-RADIANCE software pack. The findings of this study show that dimensions corresponding to a WI (Well Index) = 0.5 result in better daylighting performance inside the rooms. Moreover, it shows that increasing the number of wall surfaces on the courtyard surface tend to engender a noticeable improvement of the room’s daylighting.