This forum is closed. Thank you for your contributions.
Set your custom master page as default master page & check the checkbox for Reset all subsidies to inherit this Site Master Page setting.
Please check the below threads for reference
http://social.technet.microsoft.com/Forums/sharepoint/en-US/4af5c26b-8bf1-4d8d-be31-c78d1bf8fc05/moss2007-masterpage-to-subsite?forum=sharepointgenerallegacy
http://social.msdn.microsoft.com/Forums/sharepoint/en-US/90f40683-a797-4fe6-a92e-479b6a42b605/how-can-i-apply-customized-thememasterpage-to-a-subsite
My Blog- http://www.sharepoint-journey.com| If a post answers your question, please click Mark As Answer on that post and Vote as Helpful