By simple codeposition of Mg and B at room temperature we have previously succeeded in preparing a thick amorphous ($a$-)Mg_x_B_1-x_ film ($x\approx 0.33$) with high resistivity which exhibits superconductivity at T_c_=6.1 K. Here, we study the effects of heating on superconducting properties of the a-Mg_x_B_1-x_ film to seek the possibility to increase T_c_. We find that, by simply heating the film in vacuum up to 750 K for 10 min, both T_c_ and the critical field B_c_at 2 K increase from 6 to 9 K and 4 to 8 T, respectively, while the resistance $R$ increases slightly. The results raise basic questions concerning the roles of disorder on $R$, T_c_, and B_c_ in amorphous superconducting films. Also, we discuss whether different interpretations of the data may be possible.
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