Sir I resolved it.
Below code are working fine and tested.
Thanks for your suggestions.
config.Routes.MapHttpRoute(
name: "GetuserCredentialByID",
routeTemplate: "api/userCredentials/GetuserCredentialByID/{user_id}/{user_password}",
defaults: new { controller = "userCredentials",action = "GetuserCredentialByID", user_id = RouteParameter.Optional, user_password = RouteParameter.Optional }
);