The indoor and outdoor particulate concentration and its chemical composition in six monitoring stations and selected residential dwellings were studied in May 2010 in Kathmandu valley. The six stations were strategically located to measure the air quality of the valley viz. -urban roadside, urban hospital, urban residential, urban background and valley background. Particulate concentration was collected for 12 hours from 8 am to 8 pm. The outdoor PM10 concentrations at the urban roadside, urban hospital and urban residential stations were found to be higher. The PM10 concentrations at the urban roadside station exceeded the National Ambient Air Quality Standard (NAAQS). The indoor PM10 concentrations in the urban background and valley background stations were higher due to burning of biomass for cooking. Except for urban roadside station, the I/O ratios for all the sites were greater than 1 which indicates the presence of indoor sources. The average outdoor organic carbon (OC) and elemental carbon (EC) were 30.1 μg/m3 and 11.1 μg/m3 respectively. OC and EC were correlated (R2= 0.88, outdoors), likely indicating common sources for both species, as well as for the precursors that led to the formation of secondary organic carbon (SOC). The highest average outdoor calcium ion (Ca+) concentration of 22.5 μg/m3 at the urban roadside station indicates the presence of suspended roadside dust particles. High concentrations of Potassium ion (K+) observed in the urban and valley background stations (8.5 μg/m3 and 8.3 μg/m3 respectively) show the excessive use of biomass burning.