Fig. 3 demonstrates the transverse cross-section of the MA1 wire with HT at 650 oC for 2h.  No distinguished layer was observed in the MA1 sample. The amount of unreacted B powder for MA1 sample was greatly decreased. In addition,  a large amount of residual Mg present in MA1 sample, as shown in Fig. 4. Fig. 5 illustrates the residual Mg and unreacted B particle still exist in the MA4 sample with HT for 6 h at 650 oC. This means the added Mg powder is hard to react with B powder at 650 oC. As HT temperature was increased to 675 oC, the added Mg powder was fully reacted with B  within one hour. Therefore, no residual Mg can be observed in MA4 sample. As shown in Fig. 6, the reaction between added Mg and B also induced the formation of voids in MgB2 layer due to the melting of Mg powder and volume shrinkage. Consequently, MA4 sample has a small density of unreacted B and a high porosity for MgB2 layer.