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The x-ray tube using carbon nanotubes (CNTs) have attracted much attention as an electron source because of their excellent electron emission, micro-focusing, and low threshold voltage [1,2]. To realize a micro-focused x-ray tube using a CNT-based cold cathode, the emittance of electron beam should be decreased as small as possible, but this manner may give rise to reduce the emission current level, simultaneously. Furthermore, the field-emission behavior of CNTs strongly depends on their structural properties such as morphologies, diameters, lengths, and defects as well as on the contact with substrates especially when they are grown on an apex of a sharpened substrate.In this study, we first present the experimental results that regard the direct growth of CNTs on a conical-type tungsten (W) tip and their electron emission properties, in terms of the catalyst materials such as a nickel (Ni) and a cobalt (Co) as well as their composition ratios. Sharpening of W-tips was done by electrochemical etch and their diameters were controlled to range from 5 µm down to sub-µm. The co-sputtering method using Ni and Co targets was used to deposit the catalyst layers with various composition ratios of Ni and Co metals. The detailed conditions related to the co-sputtering process of the Ni and Co catalysts are listed in Table 1, along with the sample ID's corresponding to each condition. The composition ratio of the co-sputtered catalysts was evaluated by using an energy dispersive x-ray spectroscopy (EDS, Oxford). CNTs were grown by inductively coupled plasma-chemical vapor deposition (ICP-CVD). Fig.1 shows the microscopic SEM images of CNT-coated W-tips and compares them according to the composition ratio of Ni and Co catalysts. The length and density of the CNTs were also measured as a function of the co-sputtering power. It was found that the CNTs grown on a single Ni catalyst (sample A) exhibited a larger length and a lower density, compared with those grown on the composite catalyst of co-sputtered Ni and Co (samples B and C). Also, the CNTs grown on a single Co catalyst (sample D) showed a relatively small length and a high density. The variation in the length and density of CNTs due to the catalyst materials was conjectured to be related to the thermal expansion coefficient of the catalyst materials as well as their finally-etched size. Raman study was also performed to analyze the crystal quality of all the CNT samples. The relative intensity ratio (i.e., I D /I G ) of the two characteristic Raman peaks such as the G-line (around 1580 cm -1 ) and the D-line (around 1360 cm -1 ) was used as a fitting parameter for predicting the quality of CNTs. It is seen in Fig.2 that the I D /I G value was decreased, implying the better crystal quality, as the portion of Ni-incorporation in the catalyst was increased. Fig.3 shows the field-emission characteristics, that is, the plot of emission current versus applied voltage measured from all the CNT-coated W-tips. The anode-to-cathode (i.e., CNTs) distance for the measurement was...