Purpose / Context - Epidemiological studies have consistently shown that fine and course airborne particles (PM2.5 and PM10), as well as ultrafine (UF) particles measured in terms of particle number (PN) concentrations, are toxic to human health. A number of studies on particle concentrations in households were conducted worldwide; however, no such studies have so far been conducted in Vietnam. Methodology / Approach - Using two Nano-Tracers, the authors have simultaneously and continuously measured both indoor and outdoor number concentrations of UF particles at one low rise residential house and one apartment at a high rise building in Hanoi in order to quantify the concentrations and develop an understanding of factors driving them. Results - Daily average indoor and outdoor PN concentrations ranged from 14.5 to 19.8 x 103 p/cm3 and from 33.4 to 35.5 x 103 p/cm3, respectively. However, mean concentrations of indoor and outdoor PN during rush-hours were higher and increased up to the maximum of 23.1 and to 57.8 x 103 p/cm3, respectively. Key findings / Implications - Inspection of time series of particle concentration and subsequent statistic analysis showed that outdoor PN concentrations were strongly influenced by the outdoor vehicle emissions, while indoor PN concentrations were contributed by both indoor and outdoor sources. Originality – It is the first time, UF particle number concentrations outside and inside the residential houses in Hanoi were quantified. Outdoor particle concentrations were found strongly influenced by vehicle emisisions, while indoor particle levels affected by both indoor and outdoor sources.