Abstract: For ultrasonic TOFD test of weld specimen with large thickness, conventional excitation is difficult to satisfy the requirements of SNR(signal to noise ratio), detection distance and resolution. LFM(linear frequency modulation) pulse compression technique is applied to ultrasonic TOFD test in this paper. LFM excitation can improve both SNR and resolution. Firstly, appropriate pulse width and bandwidth for 10 MHz transducer were determined by LFM parameter test. Then, a slot defect hided in the depth of 3 mm in steel plate was tested by ultrasonic LFM-TOFD test and conventional ultrasonic TOFD test. The results show that LFM excitation can make diffracted waves separated obviously, and the precision reaches up to 0.01μs, compared with conventional excitation. In the case of lower exciting voltage and gain, higher SNR and resolution can be achieved.