The modulation transfer function (MTF) of an electro-optical device describes the image quality in terms of contrast as a function of spatial frequency, which is a much better way to evaluate a device. With the development of electro-optical devices, MTF becomes a popular technique in device evaluation. A system to measure the MTF of near infrared InGaAs linear focal plane arrays is built and an all-reflective Offner relay is used to image the slit onto the measured device. The Offner relay consists of two coaxial spherical mirrors, operating as a 11 imager. At the working wavelength of 1.7 μm, the Offner relay shows a nearly diffraction limited performance over an 8 mm×30 mm field, and the measured MTF at 20 lp/mm is above 0.8 everywhere in the field. Using this system, the MTF of an 8×1 InGaAs linear focal plane array (FPA), of which the nominal pixel size is 100 μm×100 μm is successfully measured. The result shows that the ratio of the standard deviation to the average of 6 repeated tests is below 2% within the cutoff frequency of 10 lp/mm. The relative uncertainty of the MTF measurement of the 8×1 linear FPA is below 4.7%.