Carbon dots (CDs) have received increasing attention as fluorescence sensors for the detection of different metal ions and organic small molecules. In this work, kappa-carrageenan (kappa-Carr)-derived carbon dots (C-CDs) were synthesized via a facile hydrothermal treatment with kappa-Carr as a carbon source. The produced C-CDs exhibited a high fluorescence with fluorescent quantum yield up to 20.6% and excellent fluorescent stability under diverse conditions (UV radiation, pH value and ionic strength). Then, the C-CDs were utilized as dual functional sensing probe to detect both Fe3+ and oxytetracycline (Otc). The fluorescence intensity of C-CDs was selectively quenched by Fe3+, in sharp contrast to dozens of other metal ions like Al3+, Ba2+, Ca2+, Co2+, Mg2+, Pb2+, Na+, Hg2+, Ni2+, Zn2+, Cd(2+)and Cu2+. Coincidentally, this fluorescence could also be selectively quenched by Otc, entirely distinct from related physiological molecules like glycine, ammonium hydroxide, glucose, urea. The prepared C-CDs could be used for label-free sensitive and selective fluorescent detector for Fe3+ in a wide concentration range of 0-500 mu M with a detection limit of 0.21 mu M and for Otc as well in the range of 0-100 mu M with a detection limit of 0.05 mu M. Moreover, the fluorescent C-CDs were successfully used for Fe3+ and Otc detection in some real samples, suggesting great application potential in fluorescence sensing of trace metal elements and antibiotics. [GRAPHICS] .