Two-photon anticorrelation is observed when laser and pseudothermal light beams are incident to the two input ports of a Hong-Ou-Mandel interferometer, respectively. The spatial second-order interference pattern of laser and pseudothermal light beams is reported. Temporal Hong-Ou-Mandel dip is also observed when these two detectors are at the symmetrical positions. These results are helpful to understand the physics behind the second-order interference of light.Ever since the second-order interference of light was first observed by Hanbury Brown and Twiss (HBT) in 1956 [1], it has been an important tool to study the properties of light [2]. The second-order interference of light has been studied with photons emitted by different kinds of sources, such as entangled photon pair source [3], two independent single-photon sources [4][5][6], laser and single-photon source [7], laser and entangled photon pair source [8], two lasers [9][10][11], two thermal sources [12][13][14][15][16][17], etc. Many interesting results were obtained from those studies. For instance, Hong et al. were able to measure the time separation between two photons with time resolution millions of times shorter than the resolution of the detector and the electronics [3,18]. Pittman et al. got the ghost image of an object with entangled photon pairs [19]. Bennett et al. observed Hong-Ou-Mandel (HOM) dip by feeding photons emitted by single-photon source and laser into the two input ports of a HOM interferometer, respectively [7]. The second-order interference of photons coming from laser and thermal light beams seems to have not been studied, in which, something interesting may happen. In this letter, we will experimentally study the second-order interference of laser and pseudothermal light beams in a HOM interferometer, where two-photon anticorrelation and temporal HOM dip are observed when these two detectors are at the symmetrical positions.Two-photon anticorrelation is defined as the twophoton coincidence count probability is less than the accidental two-photon coincidence count probability, which is equal to the product of these two single-photon probabilities [20]. It is convenient to employ the normalized second-order coherence function or the degree of secondorder coherence [21], g (2) (r 1 , t 1 ; r 2 , t 2 ) = G (2) (r 1 , t 1 ; r 2 , t 2 )to discuss the second-order correlation of light. Where G (2) (r 1 , t 1 ; r 2 , t 2 ) is the second-order coherence function at space-time coordinates (r 1 , t 1 ) and (r 2 , t 2 ). G(1) (r 1 , t 1 ) and G (1) (r 2 , t 2 ) are the first-order coherence functions at (r 1 , t 1 ) and (r 2 , t 2 ), respectively [22]. When g (2) (r 1 , t 1 ; r 2 , t 2 ) is greater than 1, these two photon detection events are correlated. When g (2) (r 1 , t 1 ; r 2 , t 2 ) is equal to 1, these two events are independent. When g (2) (r 1 , t 1 ; r 2 , t 2 ) is less than 1, these two events are anticorrelated. In our experiments, we are able to observe two-photon anticorrelation when these two single-photon detection event...