Does any body know relation between grease particle size with its performance?

How to measure grease particle size?

If grease is moly base then what particle size it should be & how to measure moly particle size in grease???

Perhaps you are mistaken grease particle with solid particles in grease that might be used as additives. Greases don't have a particle size as a performance measurement. So, the first question is wrongly formulated, an oxymoron, like saying "he works for the government"... nobody works in the government.

If you mean how to measure the particle size of molybdenum disulfide in greases, based on the performance, you can measure particle size, but the performance is influenced by amount of particles. If the performance is good then it is not a bigger or smaller particle, it might be because of a large amount of particles.

I am not a chemist, but try to dissolve the grease in petroleum ether, filter through a micro porous paper, and you might be able to separate the MoS2 from the grease and also the thickener. Look in the microscope to see if you can differentiate and measure particle size.

Why do you want to do this? may I ask

In certain gear applications operating at boundary conditions, and are characterized by high load (contact pressure of 100,000 psi or more) and low speed (less than 3 rpm), the size of MoS2 particles dispersed in grease does matter. At those conditions, bigger MoS2 particles get trap under the load more readily than the smaller do, where they are smeared on metal contact surfaces thus forming a sacrificial wear protective layer.