Results of study on tubular Nb3Sn magnets are presented. Magnetic flux jumping has been observed as usual. The "frozen" field distribution is closely related to the external field distribution and previous history. The distribution need not be symmetrical with respect to the mid-plane perpendicular to the cylindrical axis, as in the case of soft superconductors. This confirms the suggestion that the sintered Nb3Sn material is not a homogeneous medium.We describe a new magnet structure with which magnetic flux jumping can be avoided. With these magnets, besides the use of the iron core for trapping magnetic flux, there are also special features in both the excitation and the measurement of magnetic fields.Utilizing the tubular Nb3Sn as a piston, we have obtained a highest magnetic field of 28.5kG at 1.5°K.