The evolution of interaction and structure during the aggregation of anionic silica nanoparticles in saline water has been examined. The net attraction between charged nanoparticles with existing of salt ions is found to be significantly higher than that accounted by van der Waals force of the DLVO theory. The increase in the net attraction with salt leads to systematic transformation of nanoparticles into dimers, tetramers and higher-mers which finally results into fractal aggregate. Such multi-step aggregation is consistent with diffusion limited cluster aggregation (DLCA) fractal morphology but the fractal dimension (2.6) obtained is much larger than usually comprehended by DLCA.