Monte Carlo method is thought to be effective to solve fluid plasma equations for SOL/divertor plasmas, especially for three dimensional simulation. In the Monte Carlo algorithm based on a Lagrangian scheme, how to treat the Monte Carlo test particles at the calculation boundaries is not always trivial. In this paper, 1D diffusion equation with source terms has been solved with several different treatments of the boundaries in relatively a simple model. Comparison between the results and analytic solutions show that careful treatment of the boundary seems to be needed. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]