Replacing OMG (old magazine) to ONP (old newspaper) by raising optical property through $CaCO_3$ in-situ precipitation method in white duplex board presents cost reduction and possible drying energy saving. The strength property impairment by the presence of $CaCO_3$ could be supplemented by the fiber furnish treatment or strength polymer addition. In $CaCO_3$ in-situ precipitation of ONP, it was found from morphological study using FlowCAM, an image analyzer, that most of calcium carbonate were formed on the fines, and made the size of the fines larger. For the case of forming calcium carbonate only on the fractionated fines, the size of the fines were the biggest, and there were more clean surface areas available for bonding for the fractionated long fibers when fractionated fibers and fines were regrouped to make paper.