In the present study, we focused on several factors affecting the utility of 1‐butyl‐3‐methylimidazolium chloride (BMIMCl) for obtaining higher performance fibers. The dependence of the spinnability and tensile strength of the fibers on the zero‐shear viscosity of the spinning solutions was investigated based on differences in the molecular weight of the cellulose, pulp concentration, and the pH of BMIMCl. We demonstrated an appropriate viscosity range of 2000–4000 Pa s−1 (100 °C) for spinning dopes to obtain good spinnability and high tensile strength. The pH of the BMIMCl and the molecular weight of the cellulose clearly impacted tensile strength. The high molecular weight of cellulose contributed to high mechanical properties of the regenerated cellulose fibers. Optimizing the molecular weight and concentration of the cellulose based on the appropriate viscosity allowed us to prepare high performance cellulose fibers with a tensile strength of 1.15 GPa and a Young's modulus of 42.9 GPa.