Silica-forming peptides (SFPs) and the SFP-modified structural proteins can be used as templates for the synthesis of biosilica nanoparticles (NPs). However, such biomolecule-mediated synthesis showed limitations for the generation of NPs with sizes sub-50 nm. In this study, the SFP sequences (KPSHHHHHTGAN and KPTHHHHHHDG for Kps and Kpt, respectively) were fused to the C-terminus of the human ferritin heavy chain (Fn), resulting in the SFP moieties in the inner space of Fn (termed FncKps and Fn-cKpt). Using Fn-cSFP templates for silicification in two-phase system, the Fn-cKps@SiO2 and Fn-cKpt@SiO2 NPs were generated in mean diameters of 26 and 28 nm, respectively. Also, we employed biosilica NPs sub-50 nm for a doxorubicin (Dox) delivery system (application model). FncKpt@ SiO2 NPs exhibited a high loading efficiency compared to Fn-cKpt only (1.7-fold) and prolonged release patterns with Dox. Most importantly, the uptake of Fn-cKpt@SiO2 into cancer cells was increased so that the efficient delivery of Dox to the cell inside was observed. The uniform generation of biosilica NPs sub-50 nm obtained here is a new achievement. Together with the ideal pH-dependent drug release, the size-controlled design of biosilica will be a useful strategy for efficient delivery of chemical drugs to target cells.