Provided are organic silicon oxide fine particles which can be formed into a porous film having a dielectric constant and mechanical strength expected as a high-performance porous insulating film and having excellent chemical stability, and a preparation method thereof. Described specifically, provided are an organic silicon oxide fine particle comprising a core containing at least an inorganic silicon oxide or an organic silicon oxide and a shell containing at least an organic silicon oxide and being formed around the core by using shell-forming hydrolyzable silane in the presence of a basic catalyst; wherein of silicon atoms constituting the core or the shell, a ratio (T/Q) of a number (T) of silicon atoms having at least one bond directly attached to a carbon atom to a number (Q) of silicon atoms having all of four bonds attached to an oxygen atom is greater in the shell than in the core; and wherein the shell-forming hydrolyzable silane comprise at least a hydrolyzable silane compound having two or more hydrolyzable-group-having silicon atoms bound to each other via a carbon chain or via a carbon chain containing one silicon atom between some carbon atoms.