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The antiferromagnetic insulating ground state of the π-d molecular conductor λ-(BETS)2FeCl4 has been under intense debate for the last decades. One of the difficulties in studying this system comes from the needle-shape of its single crystal where crystallographic a+- and b+-axes are difficult to identify. We performed electron spin resonance (ESR) measurements of λ-(BETS)2FeCl4 with precise angular dependence of the g-value to study the relation between the principal axes of the g-tensor and the crystallographic axes. In the paramagnetic metal phase, the angular dependence of the g-value shows a characteristic "canine-teeth" structure owing to the strong π-d interaction. This structure develops rapidly below 150K suggesting the enhancement of the π-d spin correlation. The principal axes of the g-value are related to the crystal axes, and a simple two-step method using ESR measurements, (i) find the g-value maximum in the a+b+-plane, and (ii) find the linewidth maximum in the b+c-plane, was examined to find the easy-axis of the antiferromagnetic state. Using the two-step method, we found that antiferromagnetic resonance of the easy-axis appears below 11 K, which is higher than the metal-insulator transition temperature. [ABSTRACT FROM AUTHOR]