The effect on the physical properties of TMP handsheets by adding microfibrillated cellulose (MFC) is investigated. 26 TMP handsheet series were made with 4% addition of MFC produced by grinding or homogenization. Analysis of particle size distribution of the MFC showed that homogenization yields a bimodal distribution curve, while grinding gives a log normal distribution. The produced MFC had hydrodynamic particle sizes in the range 30-150 μm, and homogenization produced MFC with the largest particles. Addition of MFC to handsheets increased the tensile index and air resistance. MFC made by homogenization had a larger effect on the tensile index than MFC made by grinding (7-21% and 11-34% strength increase respectively), suggesting that the strengthening mechanisms of the two MFC qualities are different. The addition of MFC reduced the light scattering coefficient (a loss of 8% for the worst case), opacity and brightness of the sheets. Theoretical consideration regarding addition of fillers to maintain the optical properties showed that one still may get a 10% increase in tensile index. The use of MFC as strength enhancer in commercially produced paper is discussed.