The study of subcritical and critical systems is useful to guide decisions about design, installation and operation of some devices, mainly nuclear reactors and power plants. The information generated by these systems guides the best decisions to be made in executive project, economic viability and the safety measures to be employed in a nuclear facility. Simulating some experiments from the International Handbook of Evaluated Criticality Safety Benchmark Experiments, the MCNP5 code has been validated as an important tool to make an analysis about nuclear criticality safety. Its continuous libraries have been used. The average values and the standard deviation (SD) are evaluated. The results are very close to the values obtained by the benchmark experiments and in agreement with the criticality safety limits required by the Brazilian authorities.