This study measures the threshold level of depth of word knowledge in L2 incidental learning and reading comprehension. I administered the Word Associates Test (WAT; Read, 2004) to measure depth of word knowledge. Based on participants' scores on the WAT, they were divided into three proficiency groups: high proficiency (HP), intermediate proficiency (IP), and low proficiency (LP). As part of data collection, participants were also asked to read a short story and then complete items of the Vocabulary Knowledge Scale (VKS; Paribakht & Wesche, 1997) for the 30 chosen target words. To rate incidental vocabulary acquisition, I administered the VKS test with a repeated measure design in a period of one month. Results indicate that (a) the HPs gained more vocabulary by comparison than the IPs and the LPs, (b) the probabilistic threshold level for L2 incidental learning is from 85 to 135 out of 160 on WAT, and (c) no significant role of depth in reading comprehension has been confirmed.