Sub test()
Dim celMin As Range, celMax As Range, suprCel As Range
Set celMin = Sheets("Feuil1").Range("u21")
Set celMax = Sheets("Feuil2").Range("u17")
Set suprCel = Sheets("Feuil3").Range("a" & Rows.Count).End(xlUp)(1) 'dernière ligne non vide du tableau.
If celMin < celMax Then suprCel.EntireRow.Delete
End Sub