The target gas molecular absorption spectrum parameters especially line strengths are very important for measuring temperature and concentration with tunable diode laser absorption spectroscopy (TDLAS) technique. Usually, researchers use line strengths which come from spectrum database, like HITRAN and GEISA and HITEMP spectra database, but those database include values from the theoretical computation, as is well known, there is a big error between the theoretical value and the actual value. In order to modify the line strengths of the database, 20 ammonia spectrum absorption lines in a wavenumber range between 6611 and 6618 cm-1 are measured at different pressures by using direct tunable diode laser absorption spectroscopy (dTDLAS) technique. The measurement procedure is repeated at least 10 times at each pressure, and then average value is calculated. Voigt fitting is used to obtain all line integral area, and then the line integral area is obtained by linear fitting. The slope of the fitting straight line equals line strength. Uncertainty analysis is given for the measurements. The measured linestrength is a function of integrated area, temperature, pressure, species mole fraction and effective path length. So, the calculated linestrength uncertainties based on those parameters uncertainties, and the uncertainties of pressure, species mole fraction and effective path length are similar for all transitions with P =0.25%, =0.2%, and L =0.4%. The uncertainties of the integrated area and temperature are related to different lines, and their values come from the actual measurement. In the end, uncertainty propagation formula is used to calculate linestrength uncetainty. Uncertainties of our measured line strengths are in a 0.81%-3.33% range. Our measured line strength values are different from line strengths in the HITRAN 2012 database, and the deviations are in 0.51%-17.28% range.