Objectives: The Coronary Sinus (CS) is a widely employed structure for cannulation during electrophysiologic procedures like catheter ablation of arrhythmias, implantation of resynchronization therapy devices and percutaneous mitral valve repair. The present study aims to evaluate the variations in the Thebesian valve covering the coronary sinus ostium. Materials and Methods: The present study was conducted in randomly selected 100 human cadaveric hearts from the Department of Anatomy and autopsied bodies of Department of Forensic Medicine, Thanjavur Medical College. The presence or absence of Thebesian valve, shape of the valve, presence or absence of fenestrations in Thebesian valve were noted. Results: The Thebesian valve was present in 84% and was absent in 16%.The shape of the valve was semicircular in 84.7%, strands in 8.3% and bands in7%.The valve was fenestrated in 20.2%. An interesting variation of double crescentic Thebesian valve was present in one specimen. Conclusion: This study showed that the Thebesian valve was present in 84% of heart specimens. The prominent Thebesian valve or a fenestrated valve or a large band may be an under recognised problem interfering with cannulation. It may pose a significant challenge with regards to cannulation of coronary sinus ostium during various invasive procedures like catheter ablation for arrhythmias, implantation of resynchronization therapy devices and percutaneous mitral valve repair and coronary lead placement.