In the previous work, it is reported that the Spin-Seebeck effect (SSE), which refer to the generation of a spin current from a temperature gradient, can be enhanced by Fe interface treatment. Here, we investigated the Fe thickness (d
Fe
) dependency of spin-Seebeck voltage (V
SSE
) and mixing conductance (g
r
) in Pt/Fe/Bi:YIG/SGGG system. As a result, magnitude of V
SSE
had a peak at d
Fe
≓ 1 ML (monolayer , ≓ 0.3 mm), and also increase of g
r
was saturated at this point. It suggests that V
SSE
increase with increasing g
r
when d
Fe
is smaller than 1.0 ML. For the case in which d
Fe
is larger than 1.0ML, however, V
SSE
decreases due to a spin current decay in Fe layer with a constant g
r
. These experimental results are consistent with previous theoretical works.