The microstructures of YBCO whiskers grown by in-situ high pressure RF magnetron sputtering in Ar-O-2 atmosphere on (100) MgO substrates at a temperature range of 650-720 degrees C were characterized by transmission electron microscopy. Two characteristic, plate-like, heavily twinned crystal and whisker shapes were formed during processing; elongated-hexagonal crystals with an average aspect ratio of 2 and long rectangular whiskers with aspect ratios from 5 up to 30 (length 60 mu m). The growth direction for elongated-hexagonal crystals was [110]/[1 (1) under bar 0] and for rectangular whiskers a/b-axis. in both cases the plate normal was parallel to the c-axes. At the beginning of processing a non-uniform c-axis oriented layer was formed on the MgO substrate followed by the initiation and growth of elongated-hexagonal crystal layer. Thereafter rectangular whiskers started to grow on it.