The effect of alkaline hydrogen peroxide treatment on microfibrillar cellulose (MFC) production in an inline homogenizer (ZRI) was reported. Never-dried bleached softwood Kraft pulp (BSK) from the Stora Enso Oulu mill in Finland and never-dried dissolving cellulose (DC) from the Domsjö Fabriker AB Örnsköldsvik mill in Sweden were used. The analyses of the celluloses were carried out in three phases that included for the early refining stage, the later stage of refining and for the final product. The final refining products and the commercial MFC were visualized by FESEM (Zeiss Ultra Plus) and by AFM (VEECO multimode scanning probe microscope with a Nanoscope V controller). The results showed that the particle size of the dissolving cellulose decreased more in the early refining stage, whereas the bleached Kraft pulp was refined more in the later stage of refining. Alkaline peroxide treatment had a significant effect on the fiber width development compared to the development of alkali-treated celluloses