The present invention relates to a method of producing precipitated calcium carbonate (PCC) where a retention/strength enhancing chemical is added to milk of lime to form a mixture thereof prior to adding the mixture of the retention/strength enhancing chemical and milk of lime to paper making stock in a pipeline leading to a headbox of a paper making machine, whereafter carbon dioxide is introduced to the paper making stock, and the carbonation reaction between milk of lime and carbon dioxide is allowed to proceed in the presence of both fibers and the retention/strength enhancing chemical.