The temperature and magnetic field dependence of the magnetic properties of helimagnet Fe0.8Co0.2Si is studied by magnetic measurement and small angle neutron diffraction. It is shown that Fe0.8Co0.2Si exhibits a magnetic phase transition from conical to a new modulated spin state, phase A, just below T(N) in the field range of 0.3 kOe < H < 0.5 kOe.