CRuby adopts Mark Sweep GC Algorithm now. In my presentation, I talk about Parallel Mark Algorithm for CRuby's GC that improves Mark. Parallel mark divides a mark phase into a plurality of threads and it parallely execute. Recentlly, 8 core computers are not rare. In such an enviroment, GC will be speed up, if it distribute tasks to each core. I'll talk the parallel mark algorithm and implemention, result of benchmark.