Few complete mitochondrial genomes representing limited families in the order Hymenoptera have been sequenced. Here, we sequenced the complete mitochondrial genome of Diadegma semiclausum (Hymenoptera: Ichneumonidae). This genome is 18 728 bp long, the second largest hexapod mitochondrial genome sequenced in its entirety and that with the highest A+T content at 87.4%. Four tRNAs are rearranged compared with the ancestral arrangement. Gene rearrangement mechanisms are different among all three rearranged regions. Six tRNAs have a large variable loop, which is not found in other metazoan mitochondrial genomes. trnS(AGY) uses the abnormal anticodon TCT but trnK uses the normal CTT. The A+T-rich region is very long (2161 bp). An extremely A+T-rich (99.1%) 1515 bp tandem repeat region with three types of repeat elements is located between cox1 and cox2, and the most likely ancestral element originated from the 3' end of cox1. Independent tandem duplications followed by mutation-insertion-deletion is the best model to explain the formation of this region. These results indicate that independent evolutionary events occurred extensively, such as gene rearrangement events, gene rearrangement mechanisms, derivation of tRNA variable loops, and tandem repeat region evolutionary processes, all of which likely contribute to the diversified features of hymenopteran mitochondrial genomes.