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As one of the most popular woody species that blooms in summer, Lagerstroemia speciosa has been used abundantly in urban landscape for its excellent floral beauty. For the first time, we discovered a double-flower variant with all petaloid stamens. To understand the molecular basis of this variation, we contrasted the transcriptomes of single- and double-flower buds at three stamen development stages. In total, 73,536 unigenes were mapped and 30,714 differently expressed genes (DEGs) were identified in the tissues. We focused on the DEGs expressing in both phenotypes and investigated the association of their expression profiles with their functions in transcription pathways. Furthermore, we performed WGCNA and identified co-expressed genes with four floral homeotic genes as hubs (MADS16 , Unigene0026169; AP2 , Unigene0042732; SOC1 , Unigene0046314; AG , Unigene0056437). The expression of these hub genes has been conserved across the three developmental stages but significantly different between the two floral phenotypes. As a result, the robust transcriptional regulation of stamen petaloidy in double flowers was deduced. These findings will help to unravel the regulatory mechanisms of several specific genes, thereby providing a basis to study double-flower molecular breeding in L. speciosa. Image 1 • All stamens are converted into petals in our first discovered double-flower Lagerstroemia speciosa. • Phytohormone signals and transcription factors involve in the conversion of stamen petaloidy. • Floral homeotic genes are co-expressed and take a conservatively significant role in the regulatory network. [ABSTRACT FROM AUTHOR]