The interaction between polyolefm fibers and cementitious matrices is generally weak due to the smooth and hydrophobic surface structure of these fibers. The bonding properties of polyolefin fibers in cementitious matrices can be improved by using various surface treatment methods. The effects of low frequency (40 kHz) cold plasma treatment on polypropylene (PP) and polyethylene (PE) fibers have been investigated within the scope of this study. Argon gas has been used as plasma source at a constant treatment period (30 min) with a power of 180W. Modifications on fibers' surface structure have been monitored and chemically analyzed by using atomic force microscope (AFM) images and X-ray photoemission spectroscopy (XPS) techniques at nanoscale. Notched and unnotched prismatic cementitious composites of 40*40*160 mm have been prepared by using modified fibers at a constant fiber dosage (1% by volume). The effects of cold plasma treatment on polyolefm fibers have been investigated through the mechanical properties of these composites. Test results proved that argon plasma treatment of polyolefin fibers is considerably beneficial in terms of flexural strength and toughness when unnotched composites are considered. However, treatment of PE fibers was found more beneficial compared to that of PP fibers if the composites are notched.