where i found Ctl00 in master Page and who i can change this value?

Ctl00 is the ID of the master page and all the controls inside the master page begin with ctl00$<control name>

i want to know where i found Ctl00 in master Page and who i can change this value?

That's part of the naming schema that ASP.NET uses to ensure that no control ids inside the master page conflicts with control id on the master page. There's no way to change that.

You can safely use hard code that in the JavaScript you write (I assume you'll asking about this because you working on writing some). However, I believe ClientID will give you the id of the control with any modifications ASP.NET will make. This will allow you to programmatically generate javascript with the correct ids for controls without having to worry about how ASP.NET will modify the ids.

thanks for your help , as u say i can not changethe value of master Page ID " ctl00 " but can i see where this value stored or its hidden value ?