There is a recent interest in producing cellulose nanofibers with different surface properties from unbleached cellulose pulps for economic and environmental reasons. In the current study we investigated the use of xylanase pretreatment on two types of unbleached rice straw pulps, namely, soda and neutral sulfite, and their fibrillation to nanofibers using ultrafine grinding. The effect of xylanase pretreatment on the fibrillation progress, energy consumption, and nanofiber dimensions was studied. In addition, mechanical properties, water contact angle, water absorption, and roughness of produced nanopapers were studied. Although very thin nanofibers with a homogenous width could be isolated from both xylanase-treated and untreated pulps, the xylanase pretreatment resulted in faster fibrillation. In addition, nanopapers prepared from xylanase-treated nanofibers had better mechanical properties than those isolated from the untreated pulps. The energy consumption during fibrillation depended on the type of pulp; a slightly lower energy consumption (~ 8%) was recorded for xylanase-treated soda pulp while a higher energy consumption (~ 21%) was recorded for xylanase-treated neutral sulfite pulp compared to the untreated pulps.