>>It means that Field1 has values that cannot be converted to numeric(20,0).
>
>I have some records that have an empty values on Field1. So, that might be the reason. So, in order to achieve a complete conversion, I would have to put "0" in those empty values. Then, I can replace them with "" after.
>
>Thanks

BTW why you need Numeric(20)?
Wouldn't BIGINT work for you?
THAT is 20 bytes for record! BigInt is only 8 bytes and it is 2^63?
But you know your data better :-)

DECLARE @Test TABLE (Fld1 varchar(200), fld2 numeric(10))
INSERT INTO @Test VALUES ('1',0)
INSERT INTO @Test VALUES ('1e1',0)
INSERT INTO @Test VALUES ('1d12',0)
INSERT INTO @Test VALUES ('',0)

UPDATE @Test SET Fld2 = CAST(Fld1 as int)
WHERE ISNUMERIC(Fld1+'0e1') = 1

SELECT * FROM @Test