Industrial Control Systems (ICS) are vital for countries' smart grids and critical infrastructures. In addition to the advantages such as controlling and monitoring geographically distributed structures, increasing productivity and efficiency, ICS have brought some security problems. Specific solutions are needed to be produced for these security issues. The most important information security component for ICS is availability and the most devastating threat to this component is Denial of Service (DoS) attack. For this reason, DoS attacks carried out on Programmable Logic Controllers (PLC), an important component of ICS, have been analyzed in the paper. In the test environment where attack scenarios were implemented, real PLC devices were used in order to get the most accurate results. The destructive effects of insiders, particularly in the case of cyber attacks against ICS, in bypassing the system security measure and discovery phase also emphasized in the paper.