Analysis of skeleton is necessary for sex determination. There are several pieces of bone that are used such as pelvic bone and zygomatic arch etc. In the fieldwork of physical anthropologist and forensic physician, the skull or some pieces of the skull can be very useful. In the skull, the mastoid process is a compact and permanent process. Craniometric measurements of mastoid process are interesting parameters to distinguish sex.To study the craniometric measurements of mastoid process of dry skulls in Thais and provide a method for sex determination.One hundred normal skulls of Thais from the central region, 60 male and 40 female, were studied. The measurement of mastoid process between the three points, porion (po), mastoidale (ma), and asterion (as) were made. The three distances (po-ma, ma-as, po-as) were recorded (mm) and the mastoid triangular areas were calculated by using Heron's formula (mm2). The student's t-test and linear discriminant analysis were used for data analysis.The means of mastoid dimensions and mastoid triangular area in male are significantly larger than those of the female (p < or = 0.01). The linear discriminant analysis shows that using the mastoid process dimensions and mastoid triangular area for determining sex in Thais are convincing. The average accuracy is 67.00 to 76.90%. The better mastoid dimension is ma-as of the right mastoid process. It shows a high average accuracy at 76.90%.Means of the mastoid dimensions and mastoid triangular area of Thai skulls (whole skull or fragments) provide an accurate method for determining sex.