HVdc transmission lines extend over extensive landscapes, such as forests, prairies, and grasslands. The risk of wildfires has increased in these terrains due to the presence of combustible vegetation. One of the main issues associated with wildfires is the massive amount of smoke that typically spreads across thousands of square kilometers away from the wildfire area. Wildfire smoke is primarily composed of fine particulate matter (PM), with particle sizes dominated by PM2.5 (particulate diameter ≤ 2.5 μm) and/or PM10 (particulate diameter ≤ 10 μm). These particles can become charged and participate in gas discharge mechanisms under the high electric field around conductors of HVdc transmission lines. This thesis presents an experimental study conducted to evaluate the impact of smoke on corona discharge and breakdown characteristics of air under negative and positive dc voltage. An experimental setup in which test electrodes are placed inside a chamber, where the smoke density can be maintained at a desired level, is developed enabling the investigation of corona discharges and breakdown voltage under positive and negative dc voltages. Experiments on negative dc corona with various levels of smoke were conducted employing a needle-plane electrode configuration. The variation of characteristics of negative dc corona, with different levels of smoke in PM2.5 ranging from 0 to 6000 μg/m3) were determined and compared. It was concluded that the impact of smoke on the negative dc corona is negligible. Similarly, the effect of smoke on positive dc corona characteristics was also investigated. There is a greater particle deposition on the ground plane of the needle-plane electrode setup, caused by the positive dc corona. It was noticed that this particle deposition causes the termination of corona discharges. The particles introduced by smoke acts as a barrier to the generation of corona discharges under positive dc voltages. Additionally, the variations in the breakdown characteristics of air under various levels of smoke for positive and negative dc voltages were investigated. It was concluded that there is no significant effect of smoke on the breakdown voltage of air in a uniform electric field for the smoke density levels considered in this study.