A new Gd(III)-based metallosurfactant with double quaternary-ammonium-containing long alkyl chains was developed and subjected to the miniemulsion polymerization of vinyl monomers, giving nanosized colloids (50–110 nm) with narrow size distribution. The size of the colloid particles can be easily tuned by adjusting the feeding ratio of metallosurfactant to monomer. SEM and XPS analysis showed that the Gd(III) complexes were attached on the surface of the colloids. ICP-AES analysis further quantified the contents of Gd(III) in the miniemulsions. T1 magnetic resonance imaging (MRI) showed that these Gd(III)-loading colloids exhibited relaxivity of up to 22.77 mM−1 s−1, much higher than that of Ominiscan® (4.64 mM−1 s−1).