This paper presents an experimental study that investigates the influence of the fines on the initiation of instability of sands under cyclic loading. The materials used for this study and the experimental device are first presented, then the results of typical liquefaction tests for both loose and medium dense specimens are presented with emphasis on the exhibited behaviour. Loose samples have shown a total liquefaction phenomenon however the cyclic mobility phenomenon was initiated for medium dense samples. The results of a series of cyclic undrained tests that were done with the aid of a triaxial apparatus for different percentages of fines are also presented and analysed. The results reveal that the increase in the non-plastic fines content lead to an increase in the liquefaction resistance of sands whereas the increase in the plastic fines lead to a decrease in the resistance to liquefaction.