There is an ICE high speed service connecting Frankfurt and Berlin with no train changes required. The service runs from Frankfurt Hauptbahnhof through Hanau, Fulda, Kassel-Wilhelmshöhe, Göttingen, Hildesheim, Braunschweig and finally Wolfsburg before arriving at Berlin Hauptbahnhof.
We'd recommend this trip rather than changing in Hannover for a separate ICE service to Berlin, simply because it's easier and you don't have to worry about changing trains.
This is quite a popular route especially for business travellers so we'd recommend that you try and travel off peak.
Rail Passes and Tickets
Standard tickets for this trip are priced as above. Rail pass holders will need to pay the supplement/reservation fee (payable when reserving your seats at the station). Whilst the rail pass supplements are annoying they more than make up for the cost in the time saved over using inter city trains on routes like this.