Heteropolyacid nanoparticles (NPs) were supported on Cs-modified three-dimensionally ordered macroporous(3DOM) SiO2 and used as the catalyst for the oxidation of methacrolein (MAL) to methacrylic acid (MAA). Hydro-thermal treatment and incipient wetness impregnation were employed respectively for the Cs-modification.It was found that hydrothermal Cs-modification of 3DOM SiO2 promoted the dispersion of the supported heteropolyacid, which showed an average particle size of 5.2 nm, much smaller than that (17.6 nm) on the Cs-modified 3DOM SiO2 prepared by incipient wetness impregnation. The effects of hydrothermal treatment on the structure and catalytic performance of the catalyst were investigated. Results showed that the ion exchange between Cs+ and the surface silanol groups on 3DOM SiO2 was promoted with the increase of the hydrothermal temperature. Meanwhile, Cs-modification helped the heteropolyacid to retain intact Keggin structure, inhibiting the formation of MoO3. Highly dispersed heteropolyacid NPs with enhanced structural stability exhibited excellent selectivity to MAA in the oxidation of MAL