Brain-Computer Interfaces (BCIs) are systems that are primarily developed for use of paralyzed people. Although their main aim of use has a medical point of view, they can also be used for different aims such as entertainment and cognitive researches. Since BCI systems have specific brain potentials (P300, steady state evoked potential) and ERD/ERS (Motor Imagery), they are also flexible tools for cognitive science. In this study, an experiment was conducted with 30 participants. Each participant completed two tasks through a BCI and filled NASA-TLX forms. The results were analyzed using paired t-tests to see whether BCI tasks are significantly different in terms of creating cognitive load. The results showed that NASA-TLX scores of the BCI tasks were significantly different and these systems can be considered for estimating cognitive states studies.