Purpose: Surgical treatments for low rectal cancer require carefulconsiderations due to the low location of cancer in rectums. Successful sur-gical outcomes highly depend on surgeons' ability to determine clear distalmargins of rectal tumors. This is a challenge for surgeons in robot-assistedlaparoscopic surgery, since tumors are often concealed in rectums and roboticsurgical instruments do not provide tactile feedback for tissue diagnosis inreal-time. This paper presents the development and evaluation of an intraop-erative ultrasound-based augmented reality framework for surgical guidancein robot-assisted rectal surgery. Methods: Framework implementation consistsin calibrating the transrectal ultrasound (TRUS) and the endoscopic camera(hand-eye calibration), generating a virtual model and registering it to theendoscopic image via optical tracking, and displaying the augmented view ona head-mounted display. An experimental validation setup is designed to eval-uate the framework. Results: The evaluation process yields a mean error of0:9 mm for the TRUS calibration, a maximum error of 0:51 mm for the hand-eye calibration of endoscopic cameras, and a maximum RMS error of 0:8 mmfor the whole framework. In the experiment with a rectum phantom, our frame-work guides the surgeon to accurately localize the simulated tumor and thedistal resection margin. Conclusion: This framework is developed with ourclinical partner, based on actual clinical conditions. The experimental proto-col and the high level of accuracy show the feasibility of seamlessly integratingthis framework within the surgical workow.