A high-resolution Bitter decoration technique has been used to observe the microstructure of flux-line lattices in melt-processed YBa₂Cu₃O_y (YBCO). In particular, we studied both static and dynamic flux distributions at low (4-23 K) and high temperatures (68-69 K) after field-cooling in the presence of 150 G. We could observe the flux motion after a long deposition time. There were remarkable differences in the behaviour of the flux motion, depending on the temperature.