A clarifying agent composition comprises a diacetal powder and an organosilane treated fume silica having a pH value of 5.5 to 8, measured in a 4% w/w dispersion in 1:1 mixture of water-methanol. The diacetal has the general structure formula (I), (II), (III), (IV) or (V), wherein R1 and R2 are independently selected from the group consisting of H, C1-C4 alkyl, C1-C4 carbalkoxy, F, Cl, and Br; a is 0, 1, 2 or 3; b is 0, 1, 2 or 3 and n is 0 or 1. The organosilane treated fume silica is dispersed in the diacetal powder and being 0.05˜50% by weight the clarifying agent composition. This invention also intents to provide a new manufacturing method for preparing superfine powdery diacetal composition with high dispersion characteristic the use of said compositions for preparing polyolefin plastic article without the visible white spots on the surface of the corresponding article.