In this paper, the influence of the fiber surface sizing on development of transcrystallization, TCR, as well as the fiber-matrix interactions were analysed. Sized (CT) and unsized (CU) high-strength carbon fibers and maleated polypropylene, mPP, were used in model systems. Transcrystallization was followed by polarizing optical microscopy, in isothermal regime (Tc = 124-130degreesC). Besides the radius, distribution of the thickness and growth rate of the transcrystalline layer and the surface energetic parameters have been also determined. Lower value for Deltasigma was obtained for the system with surface treated C fibers (Deltasigma(CT) = 1,145 x 10(-7) J/cm(2)) which also contribute for their higher nucleation activity. Using FTIR microscopy and model reaction, an attempt was made to analyse the chemical interactions at fiber/matrix interface.