Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim a, b, c As Integer
        For a = 100 To 200
            c = a
            For b = 2 To c - 1
                If c Mod b = 0 Then
                Else
textbox1.text &=str(3)
                                   TextBox1.Text &= Str(c) + "是素数！！" + TextBox1.Text + Chr(13) + Chr(10)
                End If
            Next
        Next

不好意思。是str(c)不是str(3)

Dim a, b As Integer
        For a = 100 To 200
            For b = 2 To a - 1
                If a Mod b = 0 Then Exit For
            Next
            If b = a Then TextBox1.Text = Str(a) + "是素数！！" + vbCrLf + TextBox1.Text
            TextBox1.Font = New Font("", 18)
            TextBox1.ForeColor = Color.Red
        Next
这才是正确的，上面的会陷入死循环！谢谢两位