Both earth-tube systems and building thermal mass have the potential to provide desired performance levels for both indoor thermal comfort and energy saving; however, their coupling effects need to be further investigated. A model for evaluation of the combined effects of earth-tube ventilation systems and building thermal mass is proposed. Both the time-averaged indoor air temperatures and periodic temperature fluctuations are given as explicit formulas. Unlike the time lag induced by the separate use of building thermal mass, which is no longer than 6 h, the combination with an earth-tube system could extend the time lag of indoor air temperature in an annual fluctuation period by tens of days and that of a daily period by a couple of hours. It is noticeable that the building thermal mass has significant impact on the effectiveness of earth-tube systems; therefore, a small heat transfer coefficient of external envelopes helps increase the annual time lag of indoor air temperature. Using a proper combination of building thermal mass and earth tubes, indoor thermal comfort can be achieved for a building located in a region with both hot summers and cold winters without any additional cooling or heating load. (C) 2014 Elsevier B.V. All rights reserved.