With Nico Rosbergs contract set to run until the end of next year it means the teams race driver line-up is set.
Rosberg and Nakajima scored points in the Singapore Grand Prix last weekend but it has been a largely difficult year for the team. It has struggled to deliver on the promise it showed in testing and in the first race of the year at Melbourne, where Rosberg finished on the podium.
Rosberg has invariably been the quicker of the two but lost potential points on several occasions early in the season with a few crashes. Next season will be his fourth year with the team.
Nakajima has struggled to match Rosbergs pace but has picked up points on several occasions and, crucially, enjoys the backing of Williams engine supplier Toyota.
Williams has also confirmed Nico H?lkenberg will remains as test driver for 2009. H?lkenberg is currently leading the Formula Three Euroseries for ART.