The size-resolved counting efficiency of an in-house ultrafine condensation particle counter (IUCPC) being challenged by sodium chloride (NaCl) particles was experimentally determined with a reference electrometer TSI 3068B. The influence of several operational parameters including the saturator temperature (35 to 45 degrees C), the condenser temperature (10 to 25 degrees C), the capillary sample flow rate (30 to 60 mL/min) and the total flow rate (300 to 400 mL/min) on the cut-off size D-50 (particle diameter with 50% counting efficiency) was investigated using response surface design of experiments (DoE). Thirty one test points with varying operational conditions including 7 replicated test points were performed to evaluate the primary effects of operational parameters and their interactive effects on the cut-off size D-50 of the IUCPC. The results indicated that the saturator temperature and the condenser temperature had a greater impact on the cut-off size D-50 than the capillary flow rate and the total flow rate. CFD simulation demonstrated that high capillary flow rate in the IUCPC could induce a wider diffusion zone for the core aerosol flow, making more particles enter low supersaturation area near the wall. According to the ANOVA (analysis of variance), the most influential factor for the cut-off size D-50 was the condenser temperature, followed by the saturator temperature, the total flow rate and the capillary flow rate. Some interactive effects from combinations of factors on the D-50 are also significant. R-squared value of 0.9176 implied that the ultimate model for the IUCPC D-50 consistently fits the experimental results.