Melt-blown fiber webs were produced by the Exxon type die from polypropylene chip compounded with peroxide. The effect of processing conditions on the web structure and the relation between structure and physical properties of them were investigated. The processing parameters, such as the air velocity, the spin die temperature, and the die-to-collector distance (DCD) were selectively chosen within some, rangas because of the limitations of the experimental apparatus. The filtration efficiencies of the melt-blown fiberwebs prepared under these experimental conditions were also discussed. The results are shown as the following: 1) Fiber diameters are reduced remarkably as the die temperature and the air velocity are increased, however, the DCD does not affect that much. 2) The coefficient of variation(CV(%)) of the mean pore size decreases as the spin die temperature decreases. But the increase of the air velocity causes the variation of the pore size broader. 3) The finer the constituent fibers, the oil retention and filtration efficiency are increased. However, the mean pore size and the air permeability are decreased.