The relationship the between electrical properties and surface roughness (Ra) of a wet-etched silicon wafer were studied. Ra was measured by an alpha-step process and atomic force microscopy (AFM) while varying the measuring range $10{ imes}10$, $40{ imes}40$, and $1000{ imes}1000{mu}m$. The resistivity was measured by assessing the surface resistance using a four-point probe method. The relationship between the resistivity and Ra was explained in terms of the surface roughness. The minimum error value between the experimental and theoretical resistivities was 4.23% when the Ra was in a range of $10{ imes}10{mu}m$ according to AFM measurement. The maximum error value was 14.09% when the Ra was in a range of $40{ imes}40{mu}m$ according to AFM measurement. Thus, the resistivity could be estimated when the Ra was in a narrow range.