This paper represents the results of various oedometer tests by using different size of samples to compare the pre-consolidation pressure. Pre-consolidation pressure is very important parameter that shows the stress history and the consolidation behavior of cohesive soils. In this research, five different types of high plasticity clay subgrades were essentially selected for the tests. All tests were performed using the conventional oedometers with 35, 50 and 60 mm diameter rings. The purpose of this paper is to determine the effect of the sample size to the pre-consolidation pressure. The pre-consolidation pressure values were determined by using " Casagrande", "Sridharan", "Tavenas" and "Senol" methods. In all these methods, the determined preconsolidation pressures reach minimum values for 50mm diameter samples. On the other hand samples having the diameters of 35 and 60 mm give close results but comparatively higher results with respect to 50 mm samples.