using if else with eval in aspx page

If you absolutely do not want to use code-behind, you can try conditional operator for this:

<%# ((int)Eval("Percentage") < 50) ? "0 %" : Eval("Percentage") %>

That is assuming field Percentage contains integer.

Update: Version for VB.NET, just in case, provided by tomasofen:

<%# If(Eval("Status") < 50, "0 %", Eval("Percentage")) %>

<%# (string)Eval("gender") =="M" ? "Male" :"Female"%>

You can try c#

public string ProcessMyDataItem(object myValue)
 {
  if (myValue == null)
   {
   return "0 %"";
  }
   else
  {
     if(Convert.ToInt32(myValue) < 50)
       return "0";
     else
      return myValue.ToString() + "%";
  }

 }

asp

 <div class="tooltip" style="display: none">                                                                  
      <div style="text-align: center; font-weight: normal">
   Value =<%# ProcessMyDataItem(Eval("Percentage")) %> </div>
 </div>

If you are trying to bind is a Model class, you can add a new readonly property to it like:

public string FormattedPercentage
{
    get
    {
        If(this.Percentage < 50)
            return "0 %";
        else 
            return string.Format("{0} %", this.Percentage)        
     }
}

Otherwise you can use Andrei's or kostas ch. suggestions if you cannot modify the class itself