Hybrid composites made of metal-organic frameworks (MOFs) and lanthanide-doped upconversion nanoparticles (UCNPs) have attracted considerable interest for their synergistically enhanced functions in various applications such as chemical sensing, photocatalysis, anticounterfeiting and nanomedicine. However, precise assembly of MOF/UCNP hybrid composites with tunable morphologies remains a challenge due to the lack of effective synthetic methods and fundamental understanding of the growth mechanisms. Herein, we propose a modulator-directed assembly strategy to synthesize a series of ZIF-8@UCNP composites (ZIF-8 = zeolitic imidazolate framework-8). The UCNPs densely paved on the surface of ZIF-8 microcrystals and endowed the composites with intense upconversion blue emission, which were verified by steady-state/transient photoluminescence (PL) spectroscopy and single-particle imaging. Ethylenediamine (EDA) was firstly used as a modulator to fine-tune the predominant MOF facets and realized distinct morphologies of the composites. By adjusting the concentration of EDA from 0 to 25 mmol/L, the morphology of the ZIF-8@UCNP composites was tuned from rhombic dodecahedron (RD) to truncated rhombic dodecahedron (TRD), cube with truncated edges (CTE), cube, and finally a unique form of interpenetration twins (IT). The nucleation and growth process of the ZIF-8@UCNP composites was monitored by time-dependent scanning electron microscopy (SEM) images and the formation mechanism was thoroughly revealed. Furthermore, we demonstrated that the strategy for assembly of morphology-controllable ZIF-8@UCNP composites was generally applicable to various UCNPs with different sizes and shapes. The proposed strategy is expected to open up new avenues for the controllable synthesis of MOF/UCNP composites toward diverse applications.