You can use either graphite flakes or natural graphite powder as starting materials to synthesize graphene oxide. Graphite powder could be produce by grinding the graphite flakes.   

The main differences between graphite flake and graphite powder are morphology ,surface area and particle size. Particle size of graphite flakes  is more than graphite powder. The particle size will effect the quality of graphene , because smaller particles is more oxidized which means that oxidized more easily under the same conditions. Incomplete oxidation is expected with graphite flakes due to large size of flakes. For these reasons, graphite powder is mostly used  as starting material in synthesis of graphene oxide.

http://iopscience.iop.org/article/10.1088/2053-1583/3/1/014006

http://iopscience.iop.org/article/10.1088/2053-1583/3/1/014006

As Hassan said you can use both but if you want to have a good yield by flake you should apply some pre-oxidation before that and you can grounded it by salt first. based on my experience you will have bigger GO sheets by graphite flake. 

thanks all

Interesting question and good sharing.

I am following!

As the question was asked 2 years back, but I am here agreeing with the above suggested readers, also it may be useful for the new readers. I was preparing graphene oxide (GO) from Graphite powder by following Improved hummers method, good quality GO was produced (which is nothing but the proper oxidation has occurred in the graphite powder). The color of GO and broadening of the XRD peak will tell you the extent of oxidation of graphite.

Thanks.