- The objective of this study was to examine the temporal trend of the Mongolian natural vegetation phenology during 18 years between 1981 and 1999, in various ecosystems, by using two Pathfinder NOAA-AVHRR Land (PAL) products -- the Normalized Different Vegetation Index (NDVI) and Land Surface Temperature (LST). Mongolia was selected as a study area for implementing the above objectives since it enables a regional research (rather than continental or global scale). The north-south cross section is relatively short (ca. 500 km) between latitude 42o to 52o N, and covers 6 different ecosystems -- Tundra, Mountain, Forest Steppe, Steppe, Desert Steppe, and Desert. Along this cross section, precipitation ranges from more than 350 (in the north) to less than 75 mm. The entire territory consists only on natural vegetation without anthropogenic influences such as urban heat island, industry, agricultural crops etc. For estimating the growing season dynamics, an accurate determination of the beginning (greenup onset) and ending (senescence, or decline) dates of the vegetation phenology were computed. For achieving these dates four combined criteria were calculated based on the NDVI and LST datasets. Main finding of the project shows that no significant results are achieved when analyzing the entire study period over 18 years. However, when breaking the period into two sub-periods, from 1982 to 1991 and from 1992 to 1999, phenology parameters can be easily detected and results are more significant. It is shown that on the average of the entire territory, the onset starts 10 and 16 days earlier, and the decline occurs 6 and 3 days later, during the first and second sub-periods, respectively. Consequently, on the average the phenology cycle of the growing season lasts 17--19 days longer. The above-mentioned sub-periods can be visualized as a saw-tooth pattern. It is due to the eruption of Mount Pinatubo in the Philippines in June 1991, led to a global cooling of 0.5oC due to aerosols injected into the stratosphere. The three phenological parameters were interrupted by the event for 1--2 years, but both continued afterwards.