In this paper, the effect of device geometry variations on a narrow band cascoded low noise amplifier (LNA) structure performance has been studied in 30 nm gate length FinFET-based LNA operating at 10 GHz using device and mixed mode simulations in Sentaurus TCAD simulator from Synopsys. Twelve different device geometrical parameters are varied to capture their impact on LNA parameters. It is found that Tox in the range of 1.5 to 2 nm gives better noise figure (NF), Lun in the range of 2-6 nm performs better with respect to NF and Fin width Wfin in the range of 3-5 nm gives better gain performance.