Understanding spatial development of a turbulent mixing layer is essential for various aerospace applications. However, multiple factors affect physics of this flow, making it difficult to reproduce results of experiments in simulations. The current study investigates sensitivity of direct numerical simulation (DNS) of such a flow to computational parameters. In particular, effects of a time step in a temporal discretization scheme, dimensions of the computational domain, and the laminar boundary layer characteristics at the splitter plate trailing edge are considered. Flow conditions used in DNS are close to those from the experiments by Bell & Mehta (1990), where untripped boundary layers co-flowing on both sides of a splitter plate mix downstream the plate. No artificial perturbations are used in simulations to trigger the flow transition to turbulence. DNS are conducted using the spectral-element code Nek5000.