Resistance measurements have been performed to investigate the superconduct
ing state for an organic superconductor (TMET-STF)(2)BF4 with T-c=4.1 K. Th
e upper critical magnetic fields parallel and perpendicular to the conducti
on plane are determined and dimensional crossover from three dimensions (3D
) to 2D in the superconducting state is found at 3.5 K. The anisotropy of t
he critical field has a maximum at the crossover. The temperature dependenc
e of the superconducting transition width is qualitatively understood in te
rms of the thermal activated flux motion and the crossover.