Delamination of layer semiconductor materials into two-dimensional sheets can bring superior properties such as surface area, intrinsic carrier mobility and band structure. In this study, inerratic nanosized graphitic carbon nitride (g-C3N4) sheets with the lateral size of approximately 100 +/- 50 nm are conveniently fabricated by ultrasonic exfoliation of bulk g-C3N4 that derived from thermolysis of dicyandiamide after recrystallization. The nanosized g-C3N4 sheets exhibit excellently higher photocatalytic performance toward aflatoxin B-1 (AFB1) degradation under visible light than that of bulk g-C3N4. And the enhanced photocatalytic mechanisms are mainly due to the better photogenerated charges separation and larger surface area of nanosized g-C3N4 sheets, which are systematically confirmed by physicochemical and photoelectric characterization. Through identification of products structure, it is found that the initial photodegradation intermediate (C17H14O7) or main products (C14H16O4 and C12H10O4) of AFB(1) after 2 h irradiation over bulk g-C3N4 and nanosized g-C3N4 sheets are different, which may be attributed to the photoactivities and number of active groups from both catalysts during photoreaction. Therefore, this work may provide referential information for preparation of inerratic nanosized sheets and understand the related photodegradation mechanism of aflatoxin B-1.