Six La-based La-Al-Ni-Cu-(Co) alloys were subjected to a systematical study of glass formation by Bridgman unidirectional solidification at growth velocities between 0.1 and 4.82 mm/s at a temperature gradient of 15 K/mm. With increased Cu content the critical growth velocity for glass formation in La55Al25Cu20-xNix (x=0-20) alloys shows a steep minimum at 10 at.% Cu, indicating the largest glass forming ability for the La55Al25Ni10Cu10 alloy. However, replacement of Ni by Co leads to a further decrease in the critical cooling rate for the La55Al25Ni5Cu10Co5 alloy. Critical cooling rates for glass formation in the present alloys were also obtained through a study of their melting and solidification behaviors by thermal analytical measurement. The e.ect of alloying addition and the significance of reduced glass transition temperature for the glass forming ability of these alloys is discussed. c1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.