||This paper studies an Oligopoly theory, which is applied on a mobile phone market in the Czech republic in order to measure and analyze the market performance throughout the years (1995-2008). We present several approaches of measures of market performance. The chosen econometric model then gives the results. A central question we ask is whether and how much the behaviour of operators changed with an entry of new provider. We get rather expected results. Moreover, we include a critical evaluation of our model and analysis.