The influence of fuel characteristics in the formation of NO during combustion of methyl ester in oil-burner industrial boilers is presented in this paper. Under strictly controlled conditions of combustion, NO formation shows great susceptibility to biodiesel density, ignition delay, oxygen concentration, and adiabatic flame temperature. Results show that the adiabatic flame temperature affects NO formation more than the other variables, and the level of this pollutant exponentially increases with this ideal temperature for combustion. It is also shown that (a) NO formation during combustion of methyl esters linearly change with fuel density, (b) the increase of the combustion pressure reduces the effect of the density on NO formation, (c) the influence of the fuel cetane number on NO formation is intrinsic because it remains constant even when the quality of combustion is enhanced, and (d) in fuel-lean flames, oxygen of the biodiesel chemical structure increases the NO level.