In this study, first-principles calculations based on density functional theory (DFT) were performed to analyze in detail the electronic transport properties of dumbbell-shape graphene nanoribbons (DS-GNRs) and their strain dependence as a strong function of the combination of the basic structure of GNRs in the narrow segment and the structures at both ends of DS-GNRs. Then, the current-voltage characteristics (I-V characteristics) and orbital distributions of DS-GNRs were investigated to develop a highly sensitive DS-GNR-based strain sensor. By combining two GNRs with metallic and semiconducting electronic properties, a non-negligible transition layer (gradient Schottky barrier) was formed near the junction. The length of the transition layer was about five six-membered rings of carbon atoms. The formation of this transition layer is considered to be due to the exudation of the wave function from the wide segment to the narrow segment in DS-GNR. In the DS-GNR with metal-semiconductor interfaces, the strain dependence of the electronic transport properties was very complicated due to the presence of the transient Schottky barrier. On the other hand, in a DS-GNR consisting of two metallic GNRs, the Schottky barrier and the transition layer disappeared, and stable current-voltage and piezoresistive characteristics close to those of a single GNR were observed. The predicted gauge factor of DS-GNRs was larger than that of conventional metal foils (gauge factor 2–5) and close to that of conventional polysilicon (gauge factor ±30). These results indicate that DS-GNRs have the potential to produce highly sensitive and reliable strain sensors.