Hydroxyapatite (HAp) nanocrystals with different crystal sizes were deposited by the electrophoretic deposition method on the gold surface of a quartz crystal microbalance with a dissipation probe. The nanosensors formed this way were used to elucidate the adsorption mechanism of proteins with a similar pI value. The crystal sizes and the area of the a-plane affected only the adsorption amount of human serum albumin, but not that of bovine plasma fibrinogen. The viscoelastic property, Delta D/Delta f, of each absorbed layer on the nanosensors was almost constant. The protein adsorption mechanism can be explained as follows: the dissociated carboxyl groups (negative charge) of albumin were interacted with calcium ions and the hydrated amine groups (positive charge) at the alpha C domain of fibrinogen were with phosphate ions on the HAp surface.