Sub aaa()
Dim bb As String
Dim cc As String
Dim d As String
bb = Dir("C:\Documents and Settings\Administrator\桌面\需要改名" & "\*.*", vbNormal)
cc = "C:\Documents and Settings\Administrator\桌面\改名后的文件"
a = 0
Do While bb <> ""                                          '
   a = a + 1
   d = InStrRev(bb, ".")
   If Mid(bb, d, 4) = ".mp4" Then
      Name bb As cc & "\" & a & ".mp4"             '提示：文件没找到。错在哪 ？
   End If
   bb = Dir
Loop
End Sub
说明 bb 不是空的，目标文件夹中一定有*.mp4 文件苦干

[此贴子已经被作者于2015-12-25 02:20编辑过]

试试：
Sub aaa()
Dim bb As String
Dim cc As String
Dim d As String,s As String
s="C:\Documents and Settings\Administrator\桌面\需要改名\"
bb = Dir("C:\Documents and Settings\Administrator\桌面\需要改名" & "\*.*", vbNormal)
cc = "C:\Documents and Settings\Administrator\桌面\改名后的文件"
a = 0
Do While bb <> ""                                          '
   a = a + 1
   d = InStrRev(bb, ".")
   If Mid(bb, d, 4) = ".mp4" Then
      Name s & bb As cc & "\" & a & ".mp4"             '提示：bb 前面漏文件路径s
   End If
   bb = Dir
Loop
End Sub

    首先谢谢 游侠老师 的帮助，我在实际的应用中也加过目标文件的路经，也是提示"文件没找到"，因此对 Name s & bb As cc & "\" & a & ".mp4" 这句代码产生了怀疑，通过老师的帮助后肯认Name s & bb As cc & "\" & a & ".mp4" 结构没问题。经过认真的查找，发现实际应用中目标文件的路经是由Dir1 文件夹控件的.Path 属性 获取的最后不是以\"结束的。Name s "\" & bb As  cc & "\" & a & ".mp4" 成了。