A novel method was proposed to quantify the scaling formation in high-concentration direct-contact membrane distillation (DCMD). The crystallization kinetics was studied using sodium chloride as a model scalant in a polyvinylidene fluoride (PVDF) hollow-fiber module. Combined with experimental investigations, a newly established mathematical method, namely, crystallization on membrane surface (COMS) modeling, was developed and verified in terms of the membrane distillation (MD) performance and deposited crystal characteristics (median size and total number). With the experimental results of the feed concentration and DCMD performance test, the critical point of crystal deposition on the membrane surface associated with a major water flux decline was observed. On the basis of the crystal size distribution curves obtained at various operation times, the crystallization kinetic constants were determined. Combining the derived crystallization kinetic expressions with MD transport equations, the COMS modeling method was proposed to investigate the scaling formation along the operation time and quantify the critical point of a major flux decline. Correspondingly, a critical crystal size was defined and determined as 26 mu m. Furthermore, a new set of experiments under different operation conditions were carried out to verify the proposed critical size method and COMS model. The small errors (<10%) between the experimental and simulation results have confirmed the feasibility of the COMS method in quantifying the scaling process and predicting the MD performance.