We present new measurements of the diffusion constant D in standard (slab-gel) electrophoresis of DNA at fields up to 10 V/cm. Molecules investigated are bacteriophages: T4 of length 173 kbp and lambda of length 48.5 kbp cut by restriction enzyme Hindlll. We show, that D increases with the molecule length for electric field E above 5 V/cm. The results are interpreted within the geometration model.