Thermal comfort performance of three vacant naturally ventilated Royal Malaysia Police (RMP) case study lockups (LK1 in Penang, LK2 in Melaka & LK3 in Kuala Lumpur) was measured during monsoon change period from Northeast Monsoon to Southeast Monsoon. According to NGO’s report and previous studies, the lockups condition is very poor and hot which contribute to discomfort among detainees. The objectives of the study are to investigate thermal comfort performance of the lockups based on four environmental parameters (Ta, Tr, Va, & RH) through physical measurement, to predict thermal comfort performance based on operative temperature (To) and neutral operative temperature (Tneutop), and to compare the results with thermal comfort criteria recommended by ASHRAE 55 standard and previous thermal comfort studies in hot and humid climate. The results show that To and Tneutop reading of LK1 is exceeding the maximum range recommended by ASHRAE 55 and previous studies by 2% to 8% (To) and 1% (Tneutop) which categorizing LK1 condition as hot. This is mostly due to high hot airflow brought through an ineffective window opening. The results will be used as reference for improvement towards some aspects such as window opening, building finishes materials, space volume and building orientation in future lockup design.