In this paper, a new noise variance and post detection SNR estimation algorithm is proposed for MIMO OFDM systems. The estimated channel vector is employed and multiplied by a special Q matrix which comes from the QR decomposition of a partial frequency domain transform matrix. The noise components in the processed channel estimation vector are used to estimate the noise variance. The post detection SNR can be calculated out with the channel estimation results and the estimated noise variance. Simulation results show that the proposed estimator can obtain accurate estimation results. Moreover, the new estimator is not sensitive to frequency selectivity of wireless channels.