Abstract： Aim To prepare a new oral colon-specific delivery formulation and to investigate the release profile in vitro and the colon-specific delivery property in vivo in dogs. Methods Sodium 4-aminosalicylic acid was selected as the model drug. The combination of Eudragit RL30D and RS30D were used as sustained-release film, and Eudragit FS30D used as enteric film, which was expected to release drug depending on pH and time. The release profile of tablets was studied in three phosphate buffers with the pH 6.5, 7.0 or 7.4 for 12 h after a simulated gastric presoak for 2 h in 0.1 mol · L-1 HCl. The tablets were radiolabelled with 99mTc to make their release times and positions in the gastrointestinal tract be followed using a gamma camera. Results For the in vitro study, there was no drug released in 0. 1 mol ·L- 1 HCl for 2 h, and release occurred slowly when pH was above 6.5. Drug was released faster while pH was higher. For the in vivo study, the coated tablets remained intact in the upper gastrointestinal tract, and drug release began after the colonic arrival. The uncoated tablets, however, disintegrated in the stomach of the dogs rapidly. Conclusion The coating could protect the drug until the tablets reached the ascending colon, where drug was released slowly for over 10 h.