Our research focuses on computer simulations of cationic and Phosphatidylcholine (PC) containing lipid monolayers and their potential applications in drug and gene delivery. The ultimate motivation is to unravel how cationic compounds such as CTAB function for encapsulating novel DNA based drugs and other drugs e.g. protein based drugs into a delivery system. The major advantage of these drugs over traditional chemical agents is their specificity and selectivity.We employed the Berger lipid model to model lipid molecules. The Berger lipid model is based on GROMOS96 53a6 force field which has strength for describing proteins but has weakness for describing the long alkane chains of lipid molecules. The Berger lipid model did the improvement by using the Rychaert-Bellemans proper dihedral potential. Knowledge about partial charges distribution of cationic lipid molecules was retrieved from HF/MP2 level quantum chemistry calculation with the Natural Population Analysis partial charge scheme. The most important properties and behaviors of Langmuir monolayers can be obtained from the surface tension-area isotherm of the system at a given temperature. Thus a series of NVT simulations of monolayer systems of systematically varied box sizes with various cationic lipids: PC lipids ratio were done to evaluate the surface tension-area isotherms from the simulated trajectory averages. Our current results show that the cationic lipids have a tendency to stabilize the monolayers especially when the systems are more densely packed. Further investigations are on the way to fully explore other properties and behaviors of Langmuir monolayers of cationic/PC lipids.