Knowledge of visitor impacts is critical for sustainable tourism management in national parks. The focus of past tourism impact research on national parks is either on bio-physical impacts (conducted as recreation ecology research) or on social impacts (human dimensions, including environmental perception and crowding). Research integrating these two dimensions has been rarely conducted. This research aims to fill this gap through the integrative approach that attempts to understand current biophysical impacts of visitor activities in a national park, and it examines how visitors perceive these impacts. The primary objectives of this dissertation are 1) to provide a synthesis of existing of bio-physical impacts of visitor activities in the Khao Yai National Park (KYNP) and 2) to examine visitors’ perception of those impacts. Also, the factors affecting visitors’ perception are analyzed. Both qualitative and quantitative methods were used in this study. Previous impact studies conducted in KYNP were reviewed. A visitor survey was conducted between December 2008 and February 2009. The questionnaires were distributed to 628 domestic and 40 international visitors. The 38 KYNP official interviews were completed. Based on previous impact research in KYNP, the most common bio-physical impacts include soil compaction, removal of humus layer, erosion, plant damage, soil and root exposure, water quality deterioration, disturbance and feeding wildlife. Other environmental impacts include noise pollution and garbage accumulation. The results indicate that more than 30 percent of visitors do not recognize the negative results of their activities. With the exception of vegetation and water impacts, overall, visitors perceive the impacts as less severe than the actual impacts. Environmental impacts are rated differently by the KYNP officials, domestic, and international visitors. Also, significant differences were found among birders, hikers, and campers. The key factors influencing impact perceptions include income level, education level, residential location, park visitation experience, length of stay in KYNP, recreation activity, frequency of activity, group type, and group size. It is suggested that both the quality and the quantity of visitor impact research are needed to construct the body of knowledge of impacts in KYNP. A long-term impact monitoring is required to sustain the ecological integrity in KYNP.