Recent studies emphasize the role of interactions between emotions and judgment of environment to better assess affective state in animals. Diazepam offers a way to pharmacologically manipulate the affective state. This drug is generally used to reduce negative affective states, mainly by reducing fearfulness. Here we investigated whether a reduction of fearfulness through a pharmacological approach could reduce pessimistic-like judgment in lambs. We tested the effects of diazepam (0.10 mg/kg), a benzodiazepine known for its anxiolytic properties in many species, in 5-months old female lambs submitted to two experiments: the first one to assess their fear-related reactions (cortisol, cardiac and behavioural measures) to isolation and suddenness tests, and the second one to assess their putative judgment bias. In experiment 1, the lambs treated with diazepam were less frightened by isolation and suddenness than the control lambs injected with vehicle, which tended to confirm the anxiolytic effect of diazepam on sheep. In experiment 2, the lambs were first trained to perform a spatial location task to test for judgment bias in sheep. This task required a go/no-go response according to the location of a bucket in a pen, with one location being reinforced positively (with a feed reward) and the other negatively (with a fan-forced blower). Once trained, lambs (n = 16) were exposed twice (10 min and 3 h after injections) to five consecutive bucket locations: the two learnt locations plus three unrewarded, ambiguous locations set between the learnt locations. Control lambs increased their latency to approach one of the ambiguous bucket locations (P < 0.05), while treated lambs maintained the same latency to approach this location. This difference may reflect a bias in judgment in relation to fearfulness: treated lambs seemed to display a more positive judgment of an ambiguous event than control lambs. Reduction of fearfulness may thus induce a more positive affective state but this requires further investigation, with additional studies comparing the efficacy of the cognitive bias approach to other measures of affective state.