Elastic-plastic fracture toughness(J_(IC)) by ultrasonic method is evaluated in terms of width and thickness. Widths of specimen in 6061-T6 aluminum alloy are 50㎜ and 100㎜, thicknesses of those are 20㎜ and 25㎜, respectively. Elastic-plastic fracture toughness by ultrasonic method is independent of specimen thickness and side groove. Angle beam probe which are placed on the end of the compact specimen detect the maximum crack extension effectively. Comparing with elastic-plastic fracture toughness by ultrasonic method and that of unloading compliance method, J_(IC) of ultrasonic method are underestimated to that of unloading compliance method. Elastic-plastic fracture toughness of width 100㎜ specimen are underestimated to that of width 50㎜ specimen about 20%.