The potential importance of surface cohesion effects in a polyolefin reactor was demonstrated by DEM (discrete element method) simulation. Searching for a more realistic particle-to-particle interaction, for more realistic DEM simulation the force-deformation characteristics were measured for the laboratory synthesized polypropylene (PP) particles. The PP particles were synthesized in a laboratory scale micro reactor at 343 K, 0.98 MPa with TiCl3 catalyst. The product particles were then sampled and the force-deformation characteristics of individual particles were determined by a force-displacement meter newly developed for the present study with an accuracy of 0.5 nm. For displacements less than roughly 100 nm and greater than 1 mum, the force vs. displacement relationship was different because of the surface roughness effect. In the small deformation period the force F was proportional to the deformation x with a linear spring constant of 10-200 N/m. However, for a large deformation, force F became proportional to x 1.5 similar to x(3) and the equivalent linear spring constant was roughly 1000 N/m. The field emission SEM image taken after the cyclic loading showed a clear indication of flattening of particle surface roughness at around the contact point.