Min Buri Provincial Court has acquitted red-shirt supporter Amporn "Khru Khaek" Jaikorn of alleged involvement in a bomb blast in Min Buri district that killed two people in 2014.
It is the second case in which prosecutors have failed in proceedings against Ms Amporn.
Late last year the Criminal Court acquitted her of involvement in a separate bomb blast in Nonthaburi that left four people dead in 2010, the same year the pro-Thaksin United Front for Democracy against Dictatorship staged rallies against the then-Abhisit Vejjajiva government.
In Min Buri court on Thursday, the judge found the prosecution had produced no scientific evidence or witnesses which clearly implicated the Chiang Mai resident of involvement in the explosion at a car park on Rat Uthit Road on the night of March 29, 2014.
The 57-year-old was initially believed to be a woman seen meeting the another suspect, identified as Kasi Ditthanarat, at a house where explosive devices were suspected of being made.
Police investigators concluded that a home-made bomb hidden in the motorcycle ridden by the two victims had exploded prematurely.
The court found the connection between the two incidents and the defendant to be weak.
Witnesses who saw the woman could not confirm she was Ms Amporn and forensic experts did not find Ms Amporn's fingerprints at the house, the judge said.
On hearing the judgement on Thursday, Ms Amporn responded: "I have confidence in the judicial system."
She was arrested at her house in Chiang Mai's Hang Dong district on Aug 31 last year and has been denied bail.
On Nov 17 last year, the Criminal Court acquitted her of alleged involvement in an explosion at Saman Metta Mansion in Nonthaburi's Bang Bua Thong district in which four people were killed and nine others injured.
It ruled there was no clear evidence to substantiate the accusation against her. However, prosecutors have appealed the judgement.
Ms Amporn was also accused of illegal possession of an AK-47 assault rifle and 129 rounds of ammunition.
