Every site has one master page by default and a site must always have at least one master page. You can delete any master page on the Master Page Gallery page that is not set as the default master page.
To delete a master page, you must have permission to design or manage sites.
On the Site Actions menu , click Site Settings.
Note: On a site for which the Site Actions menu is customized, point to Site Settings, and then click the settings that you want to view.
In the Galleries section, click Master pages.
Point to the master page that you want to delete, click the arrow that appears, and then click Delete.
Note: To change the default master page of a site, use a Web design program that is compatible with Microsoft Windows SharePoint Services, such as Microsoft Office SharePoint Designer.