In this paper, we describe the design and implementation of a peer-to-peer based live video streaming system called Gridmedia. Gridmedia organizes the nodes into an unstructured overlay, and adopts a novel push-pull streaming mechanism to fetch data from the partner nodes. The pull mode in the unstructured overlay can work well with the high churn rate in P2P environment while the push mode can efficiently reduce the accumulated latency at user side. We also depict our practical solution of traversal over network address translators (NATs) and firewalls. Gridmedia was adopted by CCTV to broadcast the CCTV Spring Festival Gala 2005 through Internet and attracted more than 500,000 users all over the world during that night, with the maximum amount of concurrent users reaches as high as 15,239