The mortality transition is the initial phase of demographic transition by improving the population health ,increasing the ability to resist disease, and preventing premature deaths through scientific advances . In this regard, the current article examines the mortality trend in Iran using the smoothing spline method. The research data includes life expectancy estimations at different times in the country. The smoothing spline technique was used to link the estimates and plot the trend of changes in the life expectancy index, and the speed of its changes. The calculations and drawings of plots were performed using the pspline package in R software. The results showed that the mortality transition in Iran began in the early 14th century (AH) andprogressed very rapidly until the mid-1980s. After this period, this trend continued until the early 15th century (AH) at a slower pace, that is, while life expectancy is still increasing, but the increase acceleration is slowing down. This trend is expected to continue at a steady pace in the 15th century (AH). The use of the spline method in this article made it possible to model this process more accurately based on non-parametric statistics and reveal the sensitivity of the model to different estimates.