The first line of defence of steel bar corrosion in RC beams is the concrete cover. It is a common perception that thicker concrete cover provides better corrosion protection. However, a thicker concrete cover increases the depth of the flexural tension zone which may lead to larger crack width. A theoretical evaluation was carried out to study the effect of cover thickness on the flexural crack width of under-reinforced RC beams, as well as the effect of steel content. Crack width at four critical locations of the flexural tension zone were calculated, that is, at mid-height of the tension zone; at corner of the beam; at the surface nearest to the tension steel; and at the surface in between tension steel bars. The results show that the crack width is proportional to the concrete cover thickness in general. The effect of steel content on flexural crack width is location-dependent, but larger steel reinforcement ratio leads to smaller crack width in general. It was also found that the most critical location, that is, the location where maximum crack width occurs, is at the corner of the beam. However, none of the flexural crack width calculated is larger than the maximum crack width specified in the code of practice commonly used in Malaysia.