The objective of the research was to compare the nature of changes of tractive properties of tractor tyres with different external dimensions depending on the change of the vertical load level. Two diagonal tyres 9.5-24 and 31×15.5-15 with a traditional shape of a tread were selected for the research. The following tractive properties were analysed: slide, tractive force along with its components, tractive efficiency and values of the towing power and powers lost at slide and rolling resistance. Three vertical load levels of wheels were applied: 4620, 5320 and 6110 N. The research was carried out on one base (stubble field). A mobile stand aggregated with a field tractor was used for determination of tractive properties. Based on the obtained results, it was proved that both tyres similarly reacted on the change of vertical load; in both cases certain changes of tractive force and the rolling resistance were reported; whereas no differences were proved in the tractive capacities values.