The name Lorestan means "land of the Lurs", and in the wider sense consists of that part of western Iran coinciding with the province of Ilam and extending for about 650 km on a northwest to southeast axis from Kermanshah to Fars, with a breadth of 150–180 km. The terrain consists chiefly of mountains, with numerous ranges, part of the Zagros chain, running northwest to southeast. The central range has many summits which almost reach the line of perpetual snow, rising to 4000 m and more, and it feeds the headwaters of Iran's most important rivers, such as the Zayandeh rud, Jarahi, Karun, Dix, Abi, Karkheh. Between the higher ranges lie many fertile plains and low hilly, well-watered districts.
The highest point of the province is Oshtoran Kooh peak at 4,050 m. The low-lying areas being in the southern most sector of the province, are approximately 500 m above sea level.
The climate is generally sub-humid continental with winter precipitation, a lot of which falls as snow (Köppen Csa). Because it lies on the westernmost slopes of the Zagros Mountains, annual precipitation in Lorestan is among the highest anywhere in Iran south of the Alborz Mountains. At Khorramabad, the average annual precipitation totals 530 millimetres (21 inches) of rainfall equivalent, whilst up to 1270 millimetres (50 inches) may fall on the highest mountains. The months June to September are usually absolutely dry, but Khorramabad can expect 4 inches of rainfall equivalent in December and January.
Temperatures vary widely with the seasons and between day and night. At Khorramabad, summer temperatures typically range from a minimum of 12 °C (54 °F) to a hot maximum of 32 °C (90 °F). In winter, they range from a minimum of -2 °C (28 °F) to a chilly maximum of 8 °C (46 °F).