Magnesium doped potassium lithium niobate single crystal (K3Li2Nb5O15:Mg, denoted as KLN:Mg) has been grown by modified Czochralski technique. It has tetragonal tungsten bronze type structure. At room temperature, the point group is 4 mm with the lattice constants a = 1.262 nm and c = 0.398 nm. Dielectric, pyroelectric properties of a and c plates were studied in the temperature range from 10 to 700 K. The frequency dependence of dielectric constant in the frequency range 5 × 102 to 5 × 108 Hz was measured. Results reveal a paraelectric-ferroelectric phase transition near 623 K and a ferroelectric-ferroelectric phase transition near 80 K.