Surgery for rectal cancer is associated with significant side effects including wound infections, incontinence, sexual and bladder dysfunction, and long-term ostomies. Though studies have shown that patients who completely respond to preoperative treatment can safely avoid surgery, nonoperative options remain limited by the poor performance of MRI and endorectal ultrasound after initial therapy. Therefore, new imaging modalities are needed to improve posttreatment tumor assessment and enable the widespread adoption of nonoperative management in rectal cancer. An acoustic resolution photoacoustic microscope (AR-PAM) was constructed with high frequency ultrasonic transducer and near infrared laser. We performed initial phantom, and then imaged ex vivo human colorectal specimens to evaluate different AR-PAM characteristics in each tissue type (normal, untreated tumor, and treated tumor). Our data suggest that photoacoustic imaging can differentiate the distorted vasculature of rectal tumors from normal vascular patterns. However, the vascular distribution of rectal tissue in pathological complete responders showed similar distribution as the normal colorectal tissue; mucosa, submucosa and muscle layer are clearly presented in ultrasound images, while photoacoustic images have revealed that most vasculatures distribute in submucosa. Encouraged by these initial results, we are developed a high-speed scanning (1 second for 20mm B-scan) AR-PAM with laser pulse repetition rate of 1kHz for large field 3D imaging. Lateral resolution of 65μm, axial resolution of 45μm, and 8mm tissue imaging depth can be achieved.