>>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

Or you can use CASE.

... field2 = CASE WHEN LEN(field1)=0 THEN 0.0 ELSE CAST(field1 AS NUMERIC(20,0)) END

Also you can use ISNUMERIC() function or my UDF ufn_IsNumericEx to find "bad" values.