How to rename XML attribute that generated after serializing List of objects

The most reliable way is to declare an outermost DTO class:

[XmlRoot("myOuterElement")]
public class MyOuterMessage {
    [XmlElement("item")]
    public List<TestObject> Items {get;set;}
}

and serialize that (i.e. put your list into another object).

You can avoid a wrapper class, but I wouldn’t:

class Program
{
    static void Main()
    {
        XmlSerializer ser = new XmlSerializer(typeof(List<Foo>),
             new XmlRootAttribute("Flibble"));
        List<Foo> foos = new List<Foo> {
            new Foo {Bar = "abc"},
            new Foo {Bar = "def"}
        };
        ser.Serialize(Console.Out, foos);
    }
}

public class Foo
{
    public string Bar { get; set; }
}

The problem with this is that when you use custom attributes you need to be very careful to store and re-use the serializer, otherwise you get lots of dynamic assemblies loaded into memory. This is avoided if you just use the XmlSerializer(Type) constructor, as it caches this internally automatically.