Abstract: The dielectric permittivity and pyroelectric coefficient of Tb-modified Sr_0.61Ba_0.39Nb₂O₆ crystals were measured.There is a peak of the real part ε'_r1 and the imaginary part ε″_r1 around 100 K and 55 K respectively.And both the temperature coefficients peaks of the permittivity,TCε_r3 and TCε_r1,are around 55 K.Below 90 K,pyroelectricity is detected along the 100 direction.These experimental results evidence a low temperature ferroelectric-ferroelectric phase transition in these crystals.