In this work, a hydrochloric acid assisted exfoliation approach to prepare the ultrathin.carbon nitride (U-CN) nanosheet and to regulate its morphology and defects was reported. Based on moderate oxidation of hydrochloric acid, bulk carbon nitride (BCN) was just right broken into small pieces with protonation, which is beneficial to the exfoliation of BCN to U-CN nanosheets. Meanwhile, the main structure of CN do not be destroyed. Moreover, hydrochloric acid gives priority to cleave the bridging N atoms with rich elections, leading to the formation of the favorable defects and improving its photocatalytic performance. The obtained U-CN nanosheets prepared in a suitable HCl concentration exhibits enhanced photocatalytic efficiency for hydrogen evolution and Rhodamine B degradation under visible light. Such U-CN nanosheets with simple preparation, low cost, good yield and good photocatalytic activity might shed light on an acid-assisted method for developing high-performance CN photocatalysts. (C) 2017 Elsevier B.V. All rights reserved,