Abstract: This paper investigated the radial distributions of void fraction and bubble frequency and identified the flow patterns based on the lateral measurement in medium pressure boiling flow using high-temperature high-pressure single-sensor optical probe. The relations between void fraction radial distributions and flow patterns in medium pressure boiling flow were analyzed based on the experimental results. Experimental results showed that the lateral profiles of void fraction changed from near U shape towards saddle and arc as the equilibrium quality increased; local bubble frequency mainly distributed in U shape; the lateral profiles of void fraction took near U or saddle shape （local void fraction close to the wall was significantly higher than that in the core region） in bubbly flow; while in slug flow, local void fraction in the core region was only slightly lower than that in the region near the wall, and the lateral profiles took near arc shape.