A fiber-type variable optical attenuator (VOA) based on thermo-optic effect is proposed. The VOA is fabricated by etching down a certain depth and length of the cladding of optical fiber and covering with polymer with large thermo-optic coefficient. The relationship of the attenuation and the refractive index of the substitute is theoretically analyzed in terms of the mode field distribution (MFD) of the fiber. The attenuation of the VOA is measured in different substitutes of cladding. It is shown theoretically and experimentally that the attenuation varies significantly when the refractive index of the substitute has a little change, as long as the refractive index is slightly larger than that of the cladding. Moreover, the longer the etched fiber or the thinner the cladding is, the larger the attenuation of the VOA is. Therefore, this fiber-type VOA, which is substituted by a polymer with large thermo-optic coefficient and refractive index slightly higher than that of the cladding, is of large dynamic range, low insertion loss, low polarization-dependent loss, low power consumption and easily coupled or integrated with other fiber-optic components.