In this study, the effects of the process control agent (toluene) usage on the product size, morphology and conversion level of precursor powders to magnesium diborate (MgB2) was investigated. The boron and magnesium powders were first milled in Spex mill and then subjected to sintering in tube furnace. It was found that the usage of toluene at milling stage remarkably affect the final product properties. The powder size of the milled sample treated with toluene decreased to the finer sizes, especially for increasing ball to powder weight ratio and milling times. However, lower conversion values calculated for treated samples indicated the increasing contamination.