Volatile organic compounds (VOCs) in milk were investigated as quantitative markers of herbage intake (HI) at pasture. Eight Holstein cows were fed indoors with concentrate and conserved forages (grass silage, corn silage and hay) (NG), then were divided into three treatments according to the duration of access to pasture: 4 h (G4), 8 h (G8), and 20 h (G20) per day. The HIs were 4.3, 8.6, and 13.0 kg dry matter/day for the G4, G8 and G20 treatments, respectively. Milk from cows was sampled and analyzed VOCs by the steam distillation-extraction method and gas chromatography-mass spectrometry (GC-MS). From the intensity of the GC peak area, the levels of 1-phytene (3,7,11,15-tetramethyl-1-hexadecene) and 2-phytene (3,7,11,15-tetramethyl-2-hexadecene) were lowest in NG treatment and markedly increased with grazing time at pasture. With simple regression analysis on the HI to each diterpenoid, a strong correlation was found between the intensity of 1-phytene in the milk and the HI (r = 0.807, P < 0.001). 1-phytene content in milk could be useful as a quantitative marker of the HI of grazing cows.