###### Abstract  
Many species of the _Dendrobium genus_ are traditional Chinese herbal medicine and ornamental plants. Flower fragrance is one of the most important horticultural ornamental characters and plays a crucial role in the ecology, economy, and aesthetics of plants. However, the volatile constituents and key regulatory genes related to floral biosynthesis are poorly understood. In this experiment, the flowers from two species of _Dendrobium_ with high-scent smells, _Dendrobium monililiforme_ (L). Sw. (_D. moniliforme_), and light-scent smells, _Dendrobium noble_ \"H1\" (_D._ \"H1\"), were selected. The aim of this study was to explore the key gene expression profiles of floral biosynthesis by combining volatile constituent determination and transcriptome analysis in two different _Dendrobium_ species. Physiological determination results showed that 60 volatile compounds were identified in _D. moniliforme_ and 52 volatile compounds were identified in _D._ 'H1' flowers in four flowering stages, and the full bloom stage was the most complicated stage because there were 41 and 33 volatile compounds, respectively. These compounds belong to terpen, aromatics, fatty acids, nitrogenous compounds, ketones, alcohols, and alkanes, respectively. The components identified in the gynadrium and petals revealed that the petals were probably the most important sites affecting the release of volatiles. The relative content of terpene compounds was the highest, with 77.25% (_D. moniliforme_) and 50.38% (_D._ \"H1\"), respectively. Transcriptome analysis showed that differentially expressed genes (DEGs) were highly enriched in terpenoid backbone biosynthesis and that four linalool synthetase (LIS) genes were up-regulated in high-scent smell species. This study is helpful to explore the key genes of flower fragrance and provides a theoretical basis for further understanding of the regulatory molecular functions of floral synthesis and release, as well as for the cultivation of new aromatic species. Dendrobium; volatile compounds; transcriptome; floral metabolites; terpenoid 2023 2020220220220220220220220220220220220220220220220220220220220222022022022202220220222022202220222202 of _Dendrobium_[11; 12; 13; 14; 15]. Terpenoids have diverse biological functions in nature and play an essential role in plant-animal [16; 17], plant-pathogen [9; 18], and plant-plant interactions and the regulation of antagonism between organisms [19]. There are two main metabolic pathways for forming terpenoids: the mevalonate pathway (MVA pathway) and the 2C-methyl-D-erythritol-4-phosphate pathway (MEP pathway). Both pathways (MVA and MEP) are responsible for forming isoprene, which is the building block of various types of terpenes [20]. All-trans-farnesyl diphosphate (FPP) is transformed into a sesquiterpene skeleton by the MVA pathway, while geranyl diphosphate (GPP) and all-trans-geranylgeranyl diphosphate (GGPP) are produced as monoterpene and dipterpene skeletons by the MEP, respectively [21]. TPS is the final enzyme converting the precursors FPP, GPP, and GGPP to different kinds of sesquiterpene, monoterpenes, and diterpene. In addition, TPS harbors conserved motif structures such as DDxxD (an aspartate-rich motif that interacts with divalent metal ions involved in positioning the substrate for catalysis) in plants [22], but their catalytic activity and products vary greatly. Structural differences may lead to new catalytic activities, resulting in the formation of many different species of volatile terpenoids from the same substrate [23]. Monoterpenes and sesquiterpenes are the majority of volatile compounds [18]. TPSs are highly differentiated gene families that have been found in _Arabidopsis thaliana_, _Solanum Lycopersicum_, and _Vitis vinifera_[24]. In recent years, genome-wide analyses of TPSs have identified them in plants such as _Gossypium hirsutum_[25], _Brachypodium distachyon_[26], and _Liriodendron chinense_[27].