Ant optimization algorithm gets to optimal results efficiently, but it lacks initial pheromone at the beginning, which limits its further improvement. A hardware/software partitioning algorithm is presented for platform-based system-on-a-chip design. This algorithm is based on ant optimization with initial pheromone called AOwIP, and the basic ideas are: a) Use the partitioning result of reference design provided by platform-based design method as initial partition of current design, and convert the initial partition into initial pheromone for ant algorithm, and b) Based on the initial pheromone generated, the AOwIP makes use of such advantages as positive feedback and efficient convergence of the ant algorithm to search for the optimal partitioning scheme. The AOwIP adopts the system level reusing feature of platform-based design to avoid the ant algorithm's shortcoming of lacking initial pheromone. Experiments show that the AOwIP improves the performance of ant algorithm for hardware/software partitioning problems.