Todays industrys increasingly complex and automated processes cannot be optimised efficiently only using simple equations and experiences. Therefore the use of discrete simulations is on the rise, presenting real systems with digital twins. Using a simulation we can test different scenarios, even hypothetical ones, without disturbing the real system. Job shop scheduling problem represents a basic production planning problem of different sets of products. The goal of optimisation was finding the sequence of orders that equates in the shortest possible production time. Our 10 dimensional problem was optimised using pseudo-random generated sequences and using genetic algorithm. Both methods produced same optimisation result (2d 8h 44min), but we were only able to qualitatively assess the converging result, that of genetic algorithm. The algorithm has proved itself to be very useful solving simulation optimisation problems, with its fast calculations including mutations and cross-overs. It produces approximate results of the optimal solution, which is usually sufficient for real life applications. Its simplicity, speed and relative accuracy place it amongst the most used simulation optimisation algorithms.