Loop through 2 sheets

For I = 1 To 2
Sheets(I).Select
For Row = 0 To x
Z = Application.CountIf(Range,Criterium)
Sheets("sheet3").Range("B1").Offset(Row, 0).Value = Za = a + 1
Next
Next I

Without the loop for the sheets, it works out fine, and in sheet 3 is shown the results of the CountIf.
With the loop, all the results are 0.
Any idea what the problem is?

You're giving us the minimum information possible. What are you trying to do? What are the variables? Is this line a typo?

Sheets("sheet3").Range("B1").Offset(Row, 0).Value = Za = a + 1

For I = 1 To 2
Sheets(I).Select
For Row = 0 To x
Z = Application.CountIf(Rows(a),"JEF")
Sheets("sheet3").Range("B1").Offset(Row, 0).Value = Z 
a + 1
Next
Next I

typo indeed.
Well, not a lot more of information is required.
for example, I'm counting all the people named JEF in sheet 1 and 2.
In sheet 3 you can see the results.
So, I tried this For Next, to loop through the two sheets, but the results are 0.

For I = 1 To 2
Sheets(I).Select
Next I

But, if I do it without the loop for worksheets. It works perfectly, the negative point is that my code is twice as long.

Try referencing the sheet directly

Z = Application.CountIf(Sheets(I).Rows(a)

and remove the Select line. I still can't follow your code - you should define your variables.

Also, since you're asking a question here don't you think we should be the judge of how much information is needed?

Sub CountJEF()
Dim a, x, num As Integer
a = 1

'Copie the names from sheet1 to sheet3 (where the results are showed)
Worksheets("sheet1").Select
Range("A1", Range("A" & Rows.Count)).Copy Worksheets("sheet3").Range("A1")

'Making a loop to count JEF in 1 sheet
x = Range("A1").CurrentRegion.Rows.Count - 1
For Row = 0 To x
num= Application.CountIf(Rows(a), "JEF")
Sheets("Test2").Range("B1").Offset(Row, 0).Value = Num
a = a + 1
Next

End Sub

This works, the right results are showed

Sub CountJEF()
Dim a, x, num As Integer
a = 1

'Copie the names from sheet1 to sheet3 (where the results are showed)
Worksheets("sheet1").Select
Range("A1", Range("A" & Rows.Count)).Copy Worksheets("sheet3").Range("A1")

'Making a loop to count JEF in 2 sheet
x = Range("A1").CurrentRegion.Rows.Count - 1

For I = 1 to 2
For Row = 0 To x
num= Application.CountIf(Sheets(I).Rows(a), "JEF")
Sheets("Test2").Range("B1").Offset(Row, 0).Value = Num
a = a + 1
Next
Next I
End Sub

This doesn't work, all the results are 0

I hope this is enough information

Repeatedly posting the same code is not information. Post a workbook. I don't see why you can't use formulae anyway.