  With the insulating phase of \kappa-(BEDT-TTF)_2X in mind, we study a t-J
model on an anisotropic triangular lattice, where the hopping integral is t' in
one of the three directions, using a variational Monte Carlo method. By
changing the value of t'/t, we study the stability of superconducting (SC)
states with d- and d+id-wave symmetries and of an antiferromagnetic (AF) state.
As t'/t decreases from 1, the stable state immediately switches from the d+id
wave to the d wave. The AF state is stabilized from the normal spin liquid
state for t'/t\lsim 0.7 at half filling. We also take account of Nagaoka
ferromagnetism and a phase separation.
