Rok Mozic led the way for Slovenia with 25 points and Iran’s Ali Hajipour scored 15 points.
Behrouz Ataei’s team had previously lost to Japan (3-0) and Poland (3-2) and defeated China 3-1.
Iran will meet Germany, Italy, the US and the Netherlands in Week 2, slated for June 20 to 25 in Rotterdam, the Netherlands.
The Preliminary Phase campaign started on June 6 and includes matches in Nagoya (Japan), Rotterdam (the Netherlands) and Anaheim (USA).
If Iran qualifies for the VNL Finals as one of top eight teams, it will travel to Gdansk, Poland for the event from July 19 through 23.