The present invention provides an improved process for lipid nanoparticle formulation and mRNA encapsulation. In some embodiments, the present invention provides a process of encapsulating messenger RNA (mRNA) in lipid nanoparticles comprising a step of mixing an mRNA solution containing low citrate concentration and a lipid solution at an ambient temperature. Thus, the present invention provides an effective, reliable, energy-saving and cost-effective method of encapsulating mRNA into lipid nanoparticles.