Epitaxial thin films of KNbO3 were deposited on SrTiO3 (100) substrate by laser ablation. In the orthorhombic phase, the four possible domain variants in the KNbO3 (110) film growth plane were determined to be KNbO3∥SrTiO3, , , and  denoted as X+, X−, Y+, and Y−, respectively. Using a fundamental beam of 1064 nm transmitted normal to the film, the second harmonic generation (SHG) signal at 532 nm was measured and correlated to the area fractions AX+, AX−, AY+, and AY− of the four domain variants X+, X−, Y+, and Y−, respectively, in the growth plane of the film. At room temperature, the area fractions δAx=AX+−AX− and δAy=AY+−AY− were determined to be ∼3.3% and ∼2.2%, respectively. In‐situ SHG measurements revealed the phase transitions to be 210±10 °C for orthorhombic–tetragonal and 450±10 °C for tetragonal–cubic transitions. In the tetragonal phase (between 210 °C and 450 °C) the KNbO3 〈100〉∥SrTiO 3 〈100〉. The use of SHG as a sensitive, non‐destructive and real‐time probe of phase transitions and evolution of domains in ferroelectric thin films is demonstrated. © 1996 American Institute of Physics.