Rabigh Refining and Petrochemicals Company (Petro Rabigh) posted a $107m loss in the second quarter compared to $186.25m profits in the first quarter.
The first half profits have slashed 25% to $78.98m compared to $104.87m for the same first half 2010.
The company blamed a two-month closure of its facilties for the loss, as it conducted maintenance work requiring total production shutdown. Closure of the complex for maintenance from April 21 to June 30 hit sales for the comparable period of 2011 hard.
"The reason for the loss in the second quarter... is the substantial decrease in sales resulting from loss of production during the shutdown period of the complex for test and inspection maintenance," the firm said in a statement.
Operational losses for the second quarter were $103.72m, compared to a profit of $22.13m in the same quarter last year, it said.
Petro Rabigh started operations at its $10.1 billion complex in 2009. Aramco and Sumitomo Chemical each have 37.5-percent stakes in the joint venture, while the rest is publicly held.
The refinery, which caters mainly to the Saudi, European and North African markets, can process 400,000 barrels of crude per day, accounting for about 19 percent of Saudi Arabia's total refining capacity.
Petro Rabigh has a production capacity of 18 million tonnes of refined products and 2.4 million tonnes of petrochemical products.