Due to the unique physicochemical properties and membrane-permeable capacity, mesoporous silica nanoparticles (MSNs) are considered as an ideal carrier for intracellular delivery. Herein, we endeavored to address the size effect of MSNs on the cellular uptake, endosomal escape and controlled release, the key steps for the intracellular delivery. The well-ordered MSNs in the range from 55-nm to 440-nm with similar pore texture were prepared by modified base-catalyzed sol-gel method. With MC3T3-E1 model cell line, the in vitro results indicated that after 12 h cultivation, MSNs within 55 ~ 440 nm could all be internalized into the cells, and further escaped out of the endosomal compartment. The efficiency of the cellular uptake and endosomal escape strongly depended on the particle size, with the best efficiencies from 100-nm MSNs. Furthermore, the MTT results indicated that these MSNs materials were all biocompatible. The controlled release experiments with hydrophobic dexamethasone and hydrophilic vitamin C as models showed that for these small-molecular drugs, the loading amount all mainly determined by the surface area of the MSNs, and the subsequent release of the drug dramatically decreased with the increasing of the particle size. By contrast, the release rate of vitamin C was much quicker than that of the dexamethasone. These findings presented here could provide new means to tailor the size of MSNs and thus to guide the design of MSNs-based intracellular delivery system. Due to the good cell biocompatibility, high cellular uptake and endosomal escape, we conjectured that the 100-nm MSNs are more favorable for the intracellular delivery of drugs in live cells.