<jats:p>Tenecin 1, an inducible antibacterial protein secreted in the larvae of <jats:italic>Tenebrio molitor</jats:italic>, has a long N‐terminal loop and common structural feature of insect defensin family corresponding to cysteine stabilized α/β motif. To study the function of the N‐terminal loop and disulfide bridges, N‐terminal loop deleted tenecin 1, reduced tenecin 1 and tenecin 1 were chemically synthesized and their activities were measured. N‐terminal loop deleted tenecin and reduced tenecin 1 did not show antibacterial activity. Circular dichroism (CD) spectroscopy data revealed that the α‐helical content of tenecin 1 and the other proteins increased in the presence of 50% (v/v) trifluoroethanol (TFE) and the α‐helical content of tenecin 1 was much higher than that of the other proteins in buffer with or without 50% (v/v) TFE. These results suggest that disulfide bridges are necessary for the activity structure and the N‐terminal loop plays an important role in the increase of α‐helix in the membrane mimetic environment and the activity.