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Carbon nanotubes (CNTs) have been considered as prima candidates as cold-cathode electron emitters for years due to the excellent field emission characteristics they exhibit in vacuum. This high field emission of electrons is mostly contributed due to the large aspect ratio and small radius of tip curvature of each CNT coupled with the highly conductive graphitic walls. On the other hand, the quality of carbon nanotubes is greatly dependent on the deposition conditions. Impurities may exist and affect the performance of CNTs. As a result, there is a need to study the impurity effects of CNTs on the field emission. In this work, we present the effects of impurities on the field emission characteristics of the random multi-walled-CNTs (RMW-CNT) grown in-house. The CNT samples were grown under different growth conditions with thermal chemical vapor deposition (CVD) method. Impurity ratios were determined from the SEM images and ranged from 0 to 100 percent. Field emission measurements of CNTs were carried out at room temperature in vacuum (pressure of 10-6 Torr). The turn-on field, the saturation current density, and the Fowler-Nordheim plots of each sample were analyzed based on field emission results. Influences of impurity ratios on these properties were investigated.