•Composite filler by using cellulose nanofibrils, PCC and C-starch was developed.•Paper loaded with composite fillers had higher strength properties than PCC loading.•Paper loaded with composite fillers became denser with increasing the filler content. A new concept of composite filler was developed by using cellulose nanofibrils (CNF), precipitated calcium carbonate (PCC) and cationic starch (C-starch). In this study, cellulose nanofibrils were utilized in two different ways: a PCC–CNF composite filler and a papermaking additive in sheet forming. The aim was to elucidate their effects on flocculation, filler retention and the strength and optical properties of handsheets. The highest filler retention was obtained by using the PCC–CNF composite filler in paper sheets. The paper filled with the composite fillers had much higher bursting and tensile strengths than conventional PCC loading. It was also found that the paper prepared with PCC–CNF composite fillers became denser with increasing the filler content of paper.