In this letter, we demonstrate the feasibility of 3-D photoacoustic imaging using a monolithic CMOS micromachined capacitive ultrasonic sensor. The sensing membranes are released by a post-CMOS metal etch and sealed by silicon dioxide. Nine membranes, each with an inner diameter of 60 mu m, form a single detection unit with a capacitance value of 292.5 fF. A 6-mu m carbon fiber was employed to evaluate the 3-D photoacoustic imaging capability and resolution of our sensor. A 2-D large-aperture array was emulated by 2-D mechanical scanning of a single sensing element with a scanning step of 120 mu m. A 2-D synthetic aperture focusing technique was used for image reconstruction. The measured -6-dB spatial resolutions were 181 mu m in axial and 448 mu m in lateral, respectively, at the depth of similar to 2.4 cm. The 3-D photoacoustic image of a carbon fiber phantom was successfully produced.