In this study, magnetic carbon nanodots (C-dots) were synthesized by connecting C-dots and magnetic Fe3O4 nanoparticles (so called magnetic C-dots) in seeking to understand the photocatalytic activity under visible light and the recyclable ability in wastewater treatment. All of the samples were synthesized by bottom-up procedure at reaction temperatures (Tr) of 140 degrees C and 180 degrees C with different reaction times (t(r) = 0-18 h). The results indicated that the C-dots gradually attached to Fe3O4 nanoparticles with the increase of t(r) at T-r = 140 degrees C, but suddenly approached saturation adsorption on Fe3O4 particles at T-r = 180 degrees C. Microstructural images confirmed that magnetic Fe3O4 nanoparticles were surrounded by C-dots 5-10 nm in size. Optical properties illuminated that magnetic C-dots presented a red-shifted emission at lambda = 300-450 nm attesting to their photocatalytic ability in visible light. In this study, a higher extent of degradation of the dye was noted in a larger amount of C-dots on a Fe3O4 nanoparticle surface. Methylene blue (MB) concentration can be decreased by 83% within 30-min visible light irradiation. A recyclability test evidenced that the magnetic C-dots can further photodegrade large MO concentrations by at least 10-fold or more. Therefore, magnetic C-dots exhibit good degradation ability for MB under visible light, and could be easily recycled by applying a magnetic field after photodegradation, as shown in this study. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.