If you mean "SQL Server" the product from Microsoft also known as "Microsoft SQL Server 2000" or its earlier versions, then no you can not run SQL Server on a mainfram.
However, there are many types of products that could be referred to as a "SQL Server". "SQL Server" has in recent years become synonymous with "Microsoft SQL Server" however, it also can refer to any of a number of relational database products that are accessable through SQL syntax.
If you are just looking for a relational database that is accessable via SQL and can run on a mainframe, there are a few products to choose from. I am no expert in mainframes, so I will not attempt to list them here. However, I will say that SQL Server will not work on a traditional mainframe server. It will only run on a Microsoft platform that, through the miracle of clustering, can provide the same sort of performance and load capacity as a traditional mainframe.
Hope This Helps,
Scott