This research is about the design of a video decoder using dynamic task partition approach on a heterogeneous dual-core embedded platform. For such systems, static task partition between the two cores at design time is a typical approach for application development. In this paper, we propose a runtime dynamic task partition model and implement an MPEG-4 Simple Profile video decoder using this approach on a TI OMAP 5912 platform. Comparing with a traditional mobile video decoder optimized for the same DSP core, the performance gain from dynamic task partition is 38.4% on average. More importantly, the gain is achieved with the design constraint that the implementation effort for the dynamic task partition decoder is about the same as the effort using design-time task partition model. Unlike common belief that the inter-processor communication overhead would be too high to justify intense cooperation between two heterogeneous cores, this paper shows that it is indeed beneficial to adopt dynamic task partition model on commercially available heterogeneous multi-core platforms.