A new theory has been developed for geometric variation of trigate FinFETs. This geometric variation includes both line roughness induced variation and oxide-thickness variation, which can be measured from gate capacitance and Ig current variations, respectively. Experimental results show that trigate devices are subject to serious line variations as the fin height scales up and the fin-width scales down, leading to large Ion current variation, i.e., as we increase the fin aspect-ratio, line variation becomes worse which shows an increase of the active power consumption. On the other hand, oxide-thickness variation reveals significant impacts on the off-state leakage, i.e., a rough gate oxide yields to larger static power. These valuable results provide us important guideline for the design and manufacturing of high quality 3D gate FinFETs.