A flame sheet model for NO production in multi-dimensional diesel simulation has been developed. It explicitly addresses that because of the finite computational cell resolution, the local temperature within the diffusion flame is substantially higher than the cell averaged temperatures. Thus using the latter values will under-predict NO production. The methodology uses a flame sheet library for the NO production. Computations using this model are compared to experimental data for a Cummins N14 engine. The results show that the NO production is predominantly from within the flame sheets; the bulk production accounts for less than 5% of the total production.