I wanted to observe i2c signal using an oscilloscope.

But putting a probe on the SDA line made i2c stop talking.

Just 1~2 sec after the probe touching the SDA line, SDA and SCL line went LOW and HIGH respectively.

Anybody can explain why this did happen?

Thank you

I2C is just digital signals going low and high. Scope probe shouldn't bother it. If the lines don't have pullup resistors, they may act erratically. (4.7K on SCL and SDA to 5V, or 2.7K to 3.3V, depending on the VCC level you are using). The lines are only active during a transfer tho, the rest of the time they just sit at a high level (5V, or 3.3V). Nick Gammon has some nice waveforms captured here with a logic analyzer: http://www.gammon.com.au/i2c