SEOUL, Jan. 25 (Xinhua) -- South Korea's trade terms kept worsened for 13 straight months on expensive crude oil, central bank data showed Friday.

The net terms-of-trade index for goods, which measures the amount of goods that can be imported with a unit-export proceed, stood at 92.65 in December, down 6.8 percent from a year earlier, according to the Bank of Korea (BOK).

The trade terms continued to worsen since December 2017 amid the higher global crude oil price.

Dubai crude, South Korea's benchmark, averaged 65.56 U.S. dollars per barrel in November, up 7.8 percent from a year earlier. The November oil price is reflected in the December trade terms.

The export volume index inched up 0.2 percent in December from a year ago, but the export value index declined 3.3 percent, marking the first slide since September last year.

The import volume index dipped 3.1 percent last month, but the import value index gained 0.4 percent on expensive crude oil.