The objective of this thesis is to design and construct a thickness tester system, capable of measuring the thicness of foils and thin films samples based on betaparticles backscatter principle .The developed thickness tester system consists of two parts: the first is the probe with a new geometry, that contains the extended radioactive source 90 Si), Geiger Muller detector, and the sample. While the second is the lectronic module which consists of the electronic circuits of the high voltage power supply, the inverter, the pulse shaping, the timer, and the digital counter. The constructed housing of the probe is made from aluminum in a cylindrical shape of 35 mm inside diameter, 45 mm outside diameter, and a conical front of 8 mm in length. It is also made with a variable aperture diameter to determine the thickness of samples with different areas. The design parameters of the thickness tester probe were optimized through extensive experiments on the effect of the radioactive source type and geometry, the source detector and source-target distances, and the probe The aperture diameter results of this study are summarized as below: 1- This design eliminates the necessity of a point radioactive source with thin backing by using extended radioactive source (90 Sr) with a special geometry. 2- Optimum source-detector and source-target (sample) distances are found to be 10 mm, and 5 mm respectively. 3- The effective range of aperture dimeter of the probe is found to be within 6-20 mm. Increasing t h e aperture diameter enables to test a large area of the target, therefore evaluation of The the errors in the thickness measurement will be reduced. 4 . the thickness tester system was done by measuring the thickness of thin films on different substrates on which the thickness are known (standards). The results of the tester were compared with the nominal values. The accuracy achieved by this tester is found to be 4% .5- This tester has a good sensitivity to the changes in the thickness of the target material , since it was sensitive to 3% change in the thickness of Cu foils, and 4% for Al foils, while 3%, 5%, 5%, 5%, and 8% for (Cu/EPG) , (Au/Ag) , Cd / Fe ,(Au/Ag ) , and Sn Pb /Ni standard thin films respectively. 6 .The developed thickness tester is proved, a compact, portable, and accurate method for thickness measurement of thin films and foils samples 7-The smallest thicknesses measured were 9.62 μm, 110 μm, , 20.3 μm, 7 . 8 μm, 4 . 2 μm, 2.95μm, and 6 . 7 μm for Al, and Cu foils, and (Cu/EPG) (Au,ag) Sn Pb /Ni, (Au/Ag), and Cd/Fe standard.