Large female pelecinid wasps, elegant and beautiful with uniquely elongated metasoma, are found only in North, Central, and South America. These distinctive wasps are “living fossils” in a relict family of Pelecinidae. Recent discoveries of well preserved pelecinid fossils from the Middle Jurassic (165 Mya) to the Early Cretaceous (125 Mya) of northeastern China, of which Megapelecinus changi gen. et sp. nov. and Megapelecinus nashi, sp. nov. are described herein, provide strong evidence for morphological changes and evolution of pelecinids, such as body size, antennae and wing venation, over the past ≈165 million years. A phylogenetic analysis using 22 morphological characters, 12 extinct pelecinid taxa and one extant pelecinid taxon at the generic level suggests that Megapelecinus gen. nov. is the most basal pelecinid, whereas Cathaypelecinus Shih, Liu et Ren 2009, Archaeopelecinus Shih, Liu et Ren 2009, and Iscopinus Kozlov, 1974 forming a basal clade. Natural selection is demonstrated in pelecinids by the preference of larger female body size offering potentially better ovipositing capability and more efficient forewing venation with an X pattern providing potentially stronger wing structure and better flight performance for large-sized pelecinids. Furthermore, temporal and spatial analyses indicate that the most parsimonious hypothesis is that pelecinid might have originated from Northeastern China, spread to Central and Eastern Asia, and then dispersed to Americas.