As the devices are being shrunk into nanoscale, it is increasingly difficult, if not impossible, to precisely control the dopant position and number in low-dimensional nanomaterials. In this work, we have investigated doping effect on the electrical properties of n-type and p-type MoS2 monolayer, one of the representative two-dimensional layered semiconductors. We found that the sheet resistance of a nanoscale MoS2 monolayer exhibited large variation (>35%) as the dopant number or position changes, although the variation in band gap is relatively small. The variation increases as the size of MoS2 monolayer decreases. This variation in the doped MoS2 monolayer nanoribbons seems inevitable and will cause significant device-to-device variation that the integrated circuit cannot tolerate. This study also suggests that these two-dimensional semiconductors should be protected from unintentional or intentional doping if they are used in the transistors in future integrated circuits. (C) 2015 Elsevier Ltd. All rights reserved.