Backbone circularization of protein is a powerful method to improve its structural stability. In this paper, we presumed that a tight connection leads to much higher stability. Therefore, we designed circularized variants of a granulocyte-colony stimulating factor (G-CSF) with a structurally optimized terminal connection. To estimate the appropriate length of the connection, we surveyed the Protein Data Bank to find local structures as a model for the connecting segment. We set the library of local structures composed of "helix-loop-helix," subsequently selected entries similar to the G-CSF terminus, and finally sorted the hit structures according to the loop length. Two, five, or nine loop residues were frequently observed; thus, three circularized variants (C163, C166, and C170) were constructed, prepared, and evaluated. All circularized variants demonstrated a higher thermal stability than linear G-CSF (L175). In particular, C166 that retained five connecting residues demonstrated apparent Tm values of 69.4 °C, which is 8.7 °C higher than that of the circularized variant with no truncation (C177), indicating that the optimization of the connecting segment is effective for enhancing the overall structural stability. C166 also showed higher proteolytic stability against both endoprotease and exopeptidase than L175. We anticipate that the present study will contribute to the improvement in the general design of circularized protein and development of G-CSF biobetters.