A new class of magnetorheological gel that demonstrates drastic and reversible changes in dynamic modulus without using strong magnetic fields was obtained. The magnetic gel consists of carrageenan of polysaccharides and carbonyl iron particles. The magnetic gel with a volume fraction of 0.34 exhibited a reversible increase by factors of 500 of the storage modulus and 1200 of the loss modulus upon a magnetic field of 500 mT. We Postulate that the particle structural change by magnetic fields leads to the drastic change in the dynamic modulus.