The phase transformation of a zinc-2-methylimidazole-based zeolitic-imidazolate framework (ZIF), from a recently discovered ZIF-L to ZIF-8, was reported. ZIF-L is made up of the same building blocks as ZIF-8, having two-dimensional crystal lattices stacked layer-by-layer. Results indicated that the phase transformation occurs in the solid phase via the geometric contraction model (R2), a kinetic model new to ZIF. The phase transformation was monitored by means of ex situ powder X-ray diffraction, nitrogen sorption, Fourier transform infrared spectroscopy, selected-area electron diffraction, scanning electron microscopy, and in situ nuclear magnetic resonance spectroscopy. This work also demonstrates the first topotactic phase transformation in porous ZIFs, from a 2D layered structure to a 3D structure, and provides a new insight into metal–organic framework crystallization mechanisms.