Aim: To develop and psychometrically test the instrument for measuring the knowledge of traumatic brain injury of informal carers. Design Instrument development. Method: Focus group discussions were conducted among informal carers and healthcare specialists in March 2017. The content validity was determined by the mean of the item content validity index. A reliability test was performed by the Kuder‐Richardson 20 and Pearson's correlation coefficient among 40 informal carers of patients with a traumatic brain injury in the rehabilitation medicine department of a tertiary hospital from August–September 2017. Results: The final 34‐item questionnaire covers the nature of traumatic brain injury, the consequences of traumatic brain injury, the rehabilitation process, and the role of the caregiver. The item content means ranged from 0.8–1.00, and the difficulty of knowledge items ranged from 0.18–0.98. The internal consistency reliability and correlation coefficient were 0.70 and 0.84, respectively.