Abstract: Sigmoid curve was well known as a general curve to fit the growth behavior of cell, organism, or population in accordance with limitation of genetic, environmental capacity and density in their habitat. Unfortunately, many growth behaviors did not follow the basic sigmoid curve because the prerequisite assumptions were not fulfilled by actual condition. In this study, linear and nonlinear function were used to modify the exponential curve become a new growth curve which could be used in more general cases than available model nowadays. Linear modification developed basic sigmoid curve. Nonlinear functions were proposed to modify the exponential curve in this study were named after: Second order polynomial (quadratic), logarithm and second order logarithm. Basic sigmoid curve and logarithm modification on exponential curve resulted curve which always had one asymptote line so it was suitable for fitting the fiber length data at every growth ring if the samples were completely made from juvenile and mature period. If the samples came from the juvenile period alone or the researcher had not certainly sure yet, quadratic or second order logarithm modification were highly recommended to be used because those equations could result two, one, or none asymptote line. Second order logarithm was generally become the best one among others. Mathematical curve fitting on fiber length growth in this study was successfully pointed the transitional age of juvenile/mature period of Teakwood with high coefficient of determination and low standard deviation. Based on recommended mathematical method, the transitional age of Teakwood was 9-12 years old. It was more precise than visual assessment conducted by 30 undergraduate students in wood science and technology class in Bogor Agricultural University which resulted 7-15 years old. The mathematical method reduced the subjectivity and variability compared to visual assessment.