There have been many efforts to develop x-ray sources using field electron emitters instead of conventional thermionic cathodes for digital controlling of x-rays in medical imaging. Specially, portable x-ray systems need a miniature x-ray tube with less-power consumption, easy insulation of high voltage and light shielding of x-rays. Carbon nanotube (CNT) has attracted much attention as the most promising field emitter due to its geometric high aspect ratio, high physical and chemical inertness. To date, however, CNT field emitters have not been satisfactorily incorporated into a fully vacuum-sealed x-ray tube due to their instability and/or unreliability. We successfully developed a fully vacuum-sealed, miniature x-ray tube with CNT emitters for portable dental x-ray systems. The x-ray tube was designed in a triode configuration with a self-electron focusing gate and reliable CNT emitters, and was fully vacuum-sealed within a miniaturized volume of 15 mm in diagonal and 65 mm in length, very tiny and small as compared with conventional thermionic one. The nominal focal spot size of the x-ray tube is 0.4 mm with an operational tube voltage of 65 kV and a current of 3 mA, which offers quite good x-ray images of a human tooth phantom. No heating the miniature x-ray tube for electron emission leads to easy insulation of high tube voltage and light shielding of x-rays, giving a compact and light portable x-ray system. Furthermore, digital operation of the x-ray tube through an active-current control could provide a commercial lifetime along with pretty good stability.