Can I link C# (in mono-develop) with SQL Server 2005 using ADO.NET? Thanks in advance ....

Google could tell you this, if you tried to look:
http://en.wikipedia.org/wiki/Mono_%28software%29

C#, SQL Server, and ADO.NET are pieces of WINDOWS software. If you're using Linux, consider moving to Linux software and open standards, rather than trying to make Windows software 'somehow' work under Linux.

Suppose I have to make project using database (MySql) by linking it to front end made in C# using mono-develop.Can I do this or there is another better technique ??

Yes, as I said in my first post. Write the code in something other than Microsoft languages.

And you need to separate the APPLICATION from the DATABASE in your thinking. If MySQL is running, it's listening for connections on the network, and will respond. It doesn't matter if it's coming from Mac, Windows or Linux...and it doesn't matter what MySQL is running on either.

If you have a totally Windows based app/code/database now, it makes absolutely no sense to try to 'make it work' on Linux. Rewrite it to take advantage of what Linux has to offer, and it'll not only run faster, but be more stable. If you don't want to, there's not much sense in using Linux for this at all.