This paper presents a semi-fragile object-based authentication solution for MPEG4 video. To protect the integrity of the video objects / sequences, a content-based watermark is embedded into each frame in the Discrete Fourier Transform (DFT) domain before the MPEG4 encoding. A set of Angular Radial Transformation (ART) coefficients are selected as the robust features of the video objects. Error Correction Coding (ECC) is employed for watermark generation and embedding. Using this methodology, our semi-fragile authentication solution can robustly tolerate some natural object manipulations and errors (e.g. translation, scaling, rotation, lossy compression, segmentation errors, etc) while securely preventing other malicious modifications. Experimental results further demonstrate that the proposed method can achieve a good tradeoff among system robustness, security and complexity.