•Two patterns, outward and inward, designed for the evolutions of density and order.•An evolution degree R defined to normalize densification and structural ordering.•For SLCs, ordering precedes densification; for precursors, just on the contrary.•A new finding, dip-rebound phenomenon, solves which triggers crystallization.In this paper, we performed an NPT molecular dynamics simulation of crystallization process of HCP-Mg to probe the competition between densification and structural ordering. Two opposite layering patterns, i.e. outward and inward, were designed for analysis. From the perspective of solid-like cluster (SLC) itself, structural ordering always precedes densification; but from the perspective of SLC’s precursor, structural ordering always lags behind densification; the reversion occurs at the closest two liquid layers around SLC. We call it dip-rebound phenomenon. This phenomenon is a completely new finding. It resolves, to some extent, recent debate about whether densification or structural ordering triggers crystallization.Download high-res image (176KB)Download full-size image