vb 变量未定义？

因为你用了Option Explicit这语句，所以你你必须为你要用的变量定义，我看了程序，有以下变量需要定义：r,f,v.
如果你把Option Explicit这语句去掉，VB程序就不会再提示了。

Option Explicit 是强制变量定义！你要对r f v 用 dim 语句定义！或者删了Option Explicit

在Option Explicit 后面加变量定义
dim r as double
dim f as double
dim v as double

dim r as Double,f as Double,v as Double,pi as Double
Private Sub Command1_Click()
pi = 3.14
Label6.Caption = Val(InputBox("请输入圆的半径:", "输入半径"))
r = val(Label6.Caption)
f = 4 * pi * r * r
v = 4 / 3 * pi * r * r * r
Label4.Caption = f
Label5.Caption = v
End Sub

Private Sub Command2_Click()
End
End Sub