This work focuses on a surface modification of polypropylene micro-fibers having 18 μm in diameter and 12 mm in length by means of the cold low-pressure oxygen plasma treatment. The main goal is to change fiber surface properties from hydrophobic to hydrophilic and from smooth to slightly roughened and thus to ensure a strong adhesion of their surfaces with a cement matrix. As a proper indicator of realised modifications, scanning electron microscopy (an assessment of surface roughening) and a wettability measurement with demineralized water were done. Moreover, in order to establish a time-dependent stability of the chemical changes onto fiber surfaces, the wettability measurement was repeated immediately after the treatment and with a time lag equal to 1, 7 and 30 days, when fibers were exposed to standard atmospheric conditions. To asses a rate of an adhesion between fibers and the cement matrix, mechanical four-point bending tests of prismatic cement samples (CEM I 42.5 R, w/c 0,4, dimensions 40×40×160 mm) reinforced with both reference and treated fibers were performed. SEM revealed slightly roughened fiber surfaces after plasma modifications. The treated fiber wettability with water increased almost twice, compared to reference fibers. Nevertheless, the wettability fast decreased to reference values. Copyright © 2018 VBRI Press.