Using C# String.Format "{0:p0}" without the leading space before percentage sign
String.Format("{0:0%}", 0.10)
Use the NumberFormatInfo.PercentPositivePattern Property:
NumberFormatInfo numberInfo = new NumberFormatInfo();
numberInfo.PercentPositivePattern = 1;
Console.WriteLine(String.Format("{0}", 0.10.ToString("P0",numberInfo)));
If you're OK with not using Format() you could do 0.10F.ToString("0%");.