One new diamond form,the amorphous diamond (a-D) film,is prepared by filtered arc deposition.The field emission properties and mechanism of a-D film are presented,to our knowledge,for the first time.The field emission current of more than 20μA is detected below a field intensity of 20V/μm. This result is even superior to all previously reported results.The Fowler-Nordheim field- emission behavior has been observed in a-D films.The a-D films have a low work function and negative- electron affinity.