The study of thickness variation of the soft tissue in the wall of thoracic phantom to the shifting of Percentage Depht Dose (PDD) curve and Dose Profile have been done by using Monte Carlo Simulation. The linac head was designed using BEAMnrc software, whereas modeling of water and thoracic phantom using DOSXYZnrc software. The field size of beam radiation 10 x10 cm2 and the distance of source to phantom surface (SSD) 100 cm. The water phantom in cubic shape with a size 40x40x40 cm3. The thoracic phantom was designed in cubic shape, with the compositions: soft tissue, bone and lung. The thickness of the soft tissue in the wall of throcic phantom was varied 1-5 cm. This variation shows the thickness layer of the wall of the real patients. The results of comparation between monte carlo simulation and the actual measurement, show that 6 MV photon delivered by linac at Kensaras Hospital Semarang was generated using energy electron about 5.7 MeV. The variation of soft tissue thickness change the PDD curve and the dose profile curve. The variation of soft tissue thickness change the dose in the lung (depth 10 cm) at about 3,84% - 5,59% Keywords: Monte Carlo Simulation, Percentage Depht Dose (PDD), Dose profle, Tissue inhomogeneity Telah dilakukan studi pengaruh ketebalan jaringanlunak pada fantom thoraks terhadap Perubahan bentuk kurva Percentage Depht Dose (PDD) dan Dose Profile untuk berkas foton energi 6MV menggunakan Simulasi Monte Carlo. Pembuatan model treatment head linac dilakukan menggunakan program BEAMnrc, sedang permodelan fantom air maupun fantom thoraks menggunakan DOSXYZnrc. Luas lapangan yang digunakan 10x10 cm2, dengan source to skin distance (SSD) sebesar 100 cm. Fantom air didesain berukuran 40x40x40 cm3. Fantom thoraks didesain berbentuk kubus dengan komposisi jaringan lunak, tulang, dan paru-paru. Jaringan lunak bagian depan (asterior) divariasi ketebalannya 1-5 cm. Variasi ini menggambarkan ketebalan jaringan lunak dinding thoraks pasien. Hasil perbandingan antara simulasi dan pengukuran menunjukan bahwa foton 6 MV yang dihasilkan pesawat linac RS Kensaras Semarang menggunakan energi elektron pembangkit mendekati 5,7 MeV. Variasi ketebalan jaringan lunak pada dinding fantom thoraks memberikan Perubahan kurva PDD dan juga kurva profil dosis. Variasi ketebalan jaringan lunak tersebut mengubah besarnya dosis pada paru-paru (pada kedalaman 10 cm) sebesar 3,84% - 5,59%