First you have to create a field, example txtSearch and the field which you want to search in the subform let's say CompanyName. Then, goto the txtsearch field propriety, find something like update event on change and click on it.
Put the code below and modified field names and subform names to match your form.
Private Sub txtSearch_Change()
On Error GoTo Err_TrapError
Dim temp As String
temp = "*" & txtSearch.Text & "*"
If Not (txtSearch.Text = "") Then
Me.subfrmCompanySearch.Form.Filter = "CompanyName like '" & temp & "'"
Me.subfrmCompanySearch.Form.FilterOn = True
Else: Me.subfrmCompanySearch.Form.FilterOn = False
End If
End Sub
Hope this code will help.
Le888