We have developed a 3-mm diameter catheter-based photoacoustic/ultrasonic dual-mode endoscope. We successfully demonstrated its intra-instrument channel workability for the 3.7-mm diameter instrument channel of a clinical video endoscope using an in vivo swine esophagus model. Developing such a flexible and narrow diameter PAE probe compatible with current clinical video endoscopes has been an important research topic in biomedical photoacoustics because it allows a clinician to more freely apply related probe when a suspicious tissue is found. The presented system is expected to be an important technical platform for a successful clinical translation of the combined photoacoustic and ultrasonic dual-mode imaging technique to endoscopy.