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Aim or Purpose: To systematically investigate the role of HA on collagen fibril intrafibrillar mineralization and the possible mechanism. Materials and Methods: HA(Mw=5000-150000) was purchased from Aladdin company. We applied stochasticoptical reconstruction microscope (STORM) and transmission electron microscopy (TEM) to investigate the intrafibrillar mineralization of collagen fibrils. The characteristics and Ca2+ adsorption capacity of collagen was analyzed via solid surface zeta potential and inductively coupled plasma mass spectrometry. The mineralization rate of bare collagen (BC) and collagen pretreated 1.2 mg/ml HA(HA-BC) was studied by monitoring the pH changes in mineralization solution. Results: The mineralization degree of BC and HA-BC was 4.26±0.11% and 55.27±2.13%, respectively, indicating HA can significantly promote collagen intrafibrillar mineralization. STORM images clearly showed the spatial relationship of minerals present in the collagen fibrils. The collagen potential after HA modification was significantly reduced from -36.86 ± 2.26 mV to -85.90 ± 1.34 mV. Chemical analysis results show that the Ca2+ adsorption of collagen is increased significantly from 16.13 ± 1.65 mg L-1 to 22.32 ± 0.51 mg L-1 after HA modification. Time-dependent pH changes showed that the induction time in HA-BC group is 2.25 ± 0.33 h, shorter than BC group (3.09 ± 0.14 h), which implies that HA is a crystallization accelerator. Conclusions: These results confirm that HA as NCPs surrogates on intrafibrillar mineralization of collagen by reducing surface ζ potential, which facilitate attracting more Ca2+, thus providing more nucleation sites to promote ACP transformation to HAP. This project was funded by National Natural Science Foundation of China (22207097) to Wenjing Jin.