Electrical discharge machining is one of the usual and widely used machining processes for machining hard metals and alloys which has low machinability by traditional machining methods. Due to the thermoelectric nature of this process, changes in metallurgical and mechanical properties of machined surface and development of residual stresses in components are inevitable. In this research machining of Ti-6Al-4V titanium alloy is conducted by ultrasonic assisted electrical discharge machining process and the effects of ultrasonic vibration of tool on the machining efficiency, surface integrity such as surface micro-cracks, residual stress and surface hardness has been evaluated. Machined surface were imaged by scanning electron microscopy imaging to study the size and distribution of surface micro-cracks. Residual stresses along the depth of the machined surface, evaluated using Nano indentation technique and hardness of discharged surface is measured using a micro hardness measuring instrument. The results show that applying ultrasonic vibration increases electrical discharge machining process efficiency (about 90%), reduces the amount and size of surface micro-cracks, changes residual stress distribution and decreases the amount of it (average 17%); Increases of surface hardness caused by ultrasonic assisted electrical discharge machining process is 13% more than the traditional electrical discharge machining process.