Large Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 20.0
Confusion; Communication Breakdown; Situational Awareness
ATC Issue All Types; Airspace Violation All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew FLC complied w / Automation / Advisory
Airspace Structure; Company Policy; Environment - Non Weather Related; Human Factors; Procedure
I was working Final and Aircraft X checked in on the RNAV arrival from the west and I acknowledged with what runway to expect and to use caution for a similar sounding call sign on the frequency.  I then told Aircraft Y to turn left heading 070 degrees for a base turn and also cautioned that pilot of a similar sounding callsign of Aircraft X on the frequency.  By that time Aircraft Y was in the turn and I could descend to the lower Minimum Vectoring Altitude (MVA); so I transmitted to what I thought was 'Aircraft Y descend and maintain 6;500'  I heard the readback and went on with other aircraft instructions.  It turns out that I used Aircraft X callsign when I descended Aircraft Y.  [Aircraft] descended just below the 8;500 MVA and close to the 8;000 MVA so I turned their base which is away from the higher terrain and then I observed them about a mile from the 7;000 foot MVA and descending through 8;000. So I told them to maintain 7;000 feet and they read back the instruction.  I am not sure why the event happened; I must have jumbled the similar callsigns in my head.Unfortunately; I don't have a recommendation on how to prevent this; other than exercising extreme caution; which I 'thought' I already did; but apparently the law of averages got me on this one.  I will hopefully double my efforts now that this has happened and when I give the similar sounding callsign transmission... I need to slow down and really pay attention to my callsigns.  We as controllers seem to think that the similar sounding callsign warning is just for the pilots... but I have learned that it is for me also!
A Controller had two aircraft from the same company with similar sounding call signs. The Controller used the appropriate warning phraseology but then descended the wrong aircraft. The descent placed the aircraft below the Minimum Vectoring Altitude.