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Figures of merit, ZT have been measured by the improved Harman method on Bi-microwire arrays, where the diameter of the microwire is changed from 6 mum to 50 mum. The ZT values are widely distributed among the several arrays, on the contrary to those for the bulk samples. Clear correlation between the microwire diameters and ZT cannot be seen among the measured arrays. Quite large ZT of 0.287 was obtained in the 50 mumPhi Bi-microwire array and was 2.6 times larger than that in the polycrystalline bulk material. Absolute value of the Seebeck coefficient, alpha increases with increasing ZT and the thermal conductivity, kappa decreases with ZT. The relationship of the Seebeck coefficient and the thermal conductivity with ZT shows similar behavior to those in bismuth single crystals reported previously. It is concluded that the large improvement of ZT is due to the homogeneous crystal orientation in Bi-microwires constituting the array. The obtained results are important in the development of the thermoelectric devices with much higher energy conversion efficiency. It is also emphasized that the Harman method, in which the Peltier heat is utilized, is very appropriate in the study of the microscopic thermoelectric system