The crystal structure of monoclinic KGd(WO₄)₂(KGW) has been refined at room temperature by using single-crystal X-ray diffraction data. The unit-cell parameters area= 10.652 (4),b= 10.374 (6),c= 7.582 (2) Å, β = 130.80 (2)°, withZ= 4, in space groupC2/c. The linear thermal expansion tensor has been determined and the principal axes are [302], [010] and [106]. The principal axis with maximum thermal expansion (\boldalpha'_{33} = 23.44 × 10⁻⁶ K⁻¹), {\bf X}'_{3}, was located 12° from thecaxis. Undoped crystals of KGW and crystals that were partially doped by Pr, Nd, Ho, Er, Tm and Yb were grown by the top-seeding-solution growth slow-cooling method. The effect of doping on the KGW structure was observed in the cell parameters and in morphological changes. The changes in parameters follow the changes in lanthanide ionic radii. The doped crystals show {021} and {\bar{2}21} faces in addition to the {110}, {\bar{1}11}, {010}, {130} and {310} faces which basically follow the habit of the undoped KGW crystals. The development of the faces is related to the number of the most important periodic bond chains parallel to them.