We consider the problem of field emission based on carbon nanotubes. The length of these nanotubes differ from several nanometers to several hundreds of nanometers. We obtain the particle transmission function, considering the difference of potentials on the ends of nanotubes to be 𝑈 = 2 ÷ 3.5 V. Basing on the transmission function we calculate emission current. We establish the dependence of the Nordheim function on the length of nanoparticles. The limiting transition is considered for the transmission coefficient for field emission from the cathode surface in the absence of nanoparticles on it. We establish the linear dependence of current I on the field strength W and the linear dependence of function 𝐼/𝑊on the inverse value of field strength 1/W.