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Mercury pollution through water is the cause of several dangerous diseases. Various efforts have been made to reduce mercury pollution. One of them is by using sorbent. There are many ways to improve absorption efficiency, one of which is using magnetic sorbents. This study focuses on the effect of grain size and the concentration of core shell Mn0.25Fe2.75O4@SiO2 on mercury absorption efficiency. The synthesis of Mn0.25Fe2.75O4@SiO2 with different concentration of TEOS 6 and 8 mL was carried out through coprecipitation and sol-gel methods. The characterization using XRD, VSM and FTIR was carried out to determine grain size, properties and material functional groups proved that SiO2 was successfully covered on Fe3O4 Surface. While to find out the percentage of absorption was carried out with the AAS instrument. Diffraction data confirmed the presence of Fe3O4 and the amorphous of SiO2 phase. According to the Rietveld analysis for all samples demonstrated the particle size of Mn0.25Fe2.75O4 around 11-12 nm. The Mn0.25Fe2.75O4 core has superparamagnetic properties for magnetic separation, and the SiO2 shell could protect the core of being oxidized or dissolved under acid condition. FTIR results show the sample has a functional group of the main components of Fe-O at wave number 420-507 cm-1 and SiO2 at wave number 801 cm-1 (stretching) and 1078 cm-1 (bending). The results of mercury absorption test showed that the smaller the grain size and the higher the concentration of TEOS increased the percentage of mercury uptake. In addition, the absorption percentage increases with the length of absorption time given.