Large electrostrictive effect has been observed in K 0.95 Na 0.05 Ta 1−x Nb x O 3 (x=0.42, 0.43) lead-free single crystals. Cubic crystals have been grown by the top seed solution growth method. Both crystals undergo phase transition from tetragonal phase to cubic phase near room temperature. Large dielectric constant, field-induced phase transition and slimmer P–E loop produce high, purely electrostrictive effect in the crystals. The electrostrictive coefficient Q 11 , Q 12 , M 11 and M 12 of K 0.95 Na 0.05 Ta 0.57 Nb 0.43 O 3 crystal reaches as high as 0.38m 4 C −2 , − 0.10m 4 C −2 , 17.6×10 −16 m 2 V −2 and −0.46×10 −16 m 2 V −2 , respectively. The Q 11 value of the crystal is 15 times larger than that of high performance Pb-based Pb(Mn 1/3 Nb 2/3 )O 3 ceramics.