This study focused on spray combustion of ultra low sulfur diesel (ULSD) fuel under low oxygen conditions with low temperature combustion (LTC) mode in an optically accessible constant volume combustion chamber. The ambient oxygen concentration was configured as 10% and 15% to achieve low flame temperature. The ambient gas temperature varied from 800 K to 1200 K. High speed imaging of OH* chemiluminescence and natural luminosity (NL) was used to visualize the instantaneous spray combustion process. The heat release rate was analyzed using the transient combustion pressure and the flame structure was studied based on the combustion images. Results show that a higher oxygen concentration case features a shorter ignition delay and higher heat release rate. The LTC mode can be realized by decreasing the oxygen concentration and ambient temperature simultaneously and it features a longer ignition delay, a slower reaction rate, and apparently lower soot radiation heat loss. The visualization results of NL and OH* show that the high temperature reaction occurs mainly in the mid-stream and downstream of the spray combustion, but not in the region very close to the chamber wall. This study validates the LTC process by showing very weak OH* chemiluminescence signal. The results also indicate that in order to realize LTC mode, it is important to control the ambient oxygen and ambient temperature at the same time. By only reducing the ambient oxygen concentration it may not be effective to suppress soot generation. (C) 2012 Elsevier Ltd. All rights reserved.