Effects of six different forage cropping treatments, perennial ryegrass (RG), alfalfa (AL), bromegrass (BR), small burnet (SB), subterranean clover (SC) and purple crownvetch (CV), oil soil structural parameters such as, mean weight diameter (MWD), aggregate stability (AS), dispersion ratio (DR), bulk density (BD) and penetration resistance (PNTR) were determined. Forage cropping decreased the proportion of rnicroaggregates in the fractions <0.5 turn in size and increased the proportion of macroaggregates in the fractions > 1.00 mm in size. All forage cropping significantly increased organic carbon (OC) content (1.34-1.84%), MWD (0.89-1.15 mm), AS (71.76-80.01%) and decreased DR (15.30-9.76%), BD (1.45-1.28 g cm(-3)) and PNTR (2.94-1.89 MPa) compared with the fallow control treatment. The most positive effects oil soil structural parameters were generally obtained with BR treatment while the lowest effects on that were obtained with RG treatment amongst the forages. Fragmentation (D) and mass (D-m) fractal dimensions were decreased with increasing numbers of macroaggregates of the clay soil due to forage cropping. The lowest values of D and D-m were found as 2.059 and 1.582 for BR treatment while the highest values of them were found as 2.683 and 2.085 for the control treatment, respectively. Both fractal dimensions gave positive relationships with OC content, MWD, AS, and negative relationships with DR, BID and PNTR. Development of macroaggregates in clay soil by the forage cropping was represented by the fractal dimension better than mass fractal dimension and MWD. (C) 2005 Elsevier B.V. All rights reserved.