The effect of nitrogen addition on the growth rate, quality, and properties of polycrystalline diamond grown by microwave plasma assisted (MPA)CVD is investigated. Two series of experiments are performed at two different microwave power densities (40 and 110 W cm-3) using a 2.45 GHz cylindrical microwave reactor. The results show that the beneficial effect of nitrogen is more distinct at higher microwave power densities. To investigate the properties of polycrystalline diamond grown with nitrogen addition, a thick diamond disk of 50 mm diameter is grown with an addition of 50 ppm nitrogen using a 2.45 GHz ellipsoidal microwave reactor. The grown diamond disk has a thermal conductivity of 17.3 W cm-1 K-1 and dielectric loss tangent of 3.7 × 10-5 at a frequency of 170 GHz, and its parameters are suitable for application in microwave windows (e.g., gyrotron windows). Our results indicate that it is possible to achieve the increased (by a factor of 2.5) growth rates by nitrogen addition without significant degradation of diamond quality, and properties such as thermal conductivity and dielectric loss tangent.