Abstract: In this study the whole process of pollination an d fertilization in aDole was observed with a fluores．cent microscope， and the thinning mechanism of MCPB—ethyl【4一(4一chl0m一2一methyl phenoxy)butanoate]and CFA (calcium．formic acid compound)as flower thinners Was studied anatomically．It iS proved that MCPB—ethyl and CFA ale entirely diferent in interfering pollination and fertilization of apple． MCPB—ethyl spray did not interfere in the rmination of pollen，the elongation of pollen tube in style and to ovule an d fertilization，but the development of ovule Was restrained to result in fruit drop． On the contrary，CFA did not afect the development of ovule．Fruit drop Was res~ted due to injury of stigma，pollen on stigma，and polen tube in up style，and fertil—ization Was prevented by CFA treatment．

Key words: Apple, Flower thinning, Polination an d fertilization, Developme nt of ovule