Composite materials, made of graphitic carbon nitride/gadolinium and nitrogen co-doped titanium dioxide (g-C3N4/Gd-N-TiO2), have been successfully prepared by a simple and reproducible in-situ synthetic method. Specific surface area, pore size, particle morphology, crystalline structure, energy band gap, and chemical bonding mode of the materials have been characterized by spectroscopic and electron microscopic means. Photoluminescence measurement and photocatalytic degradation of methyl blue (MB) and antibiotics (gatifloxacin) test show that the composites exhibit significant enhanced photocatalytic activity. Our experimental results indicate that nanoscale heterojunctions are formed in the composites, and these junctions have played a key role in the separation and transportation of photo generated electrons and holes, and hence the effective promotion of the photocatalysis.